"This is my commanment, that you love one another as I have loved you" (Jn 15, 13)
1) The anti-Gnostic astonishment that the transcendent God has assumed a full and complete humanity replete with will and heart is at the basis of this feast. Perhaps, John Paul said it best by asking, “Could God go further in His stooping down, in His drawing near to man, thereby expanding the possibilities of our knowing Him? In truth, it seems that He has gone as far as possible. He could not go further. In a certain sense God has gone too far! Didn’t Christ perhaps become `a stumbling block to Jews and foolishness to Gentiles’ [1 Cor 1, 23]? (Crossing the Threshold of Hope, Knopf (1994) 40).
2) Since Jesus Christ is divine Person whose very to be = to for the other as pure relation to the Father, and since there is only one act of existence (esse) in Christ that dynamizes His entire Person (that includes body, soul and potencies) [see Aquinas S. Th. III, 17, 2 ad 2], this means that the human love of Christ has a divine dimension. This translates into a radicalness of self-giving that ends in dying for the other, i.e., for us.
3) Benedict XVI’s “The Paschal Mystery as Core and Foundation of Devotion to the Sacred Heart,” Towards a Civilization of Love (1981) 145-165.
a) The body is the person expressed: “(T)he body is not just `there,’ having a merely external relationship to the spirit; rather, the body is the self-expression and `image’ of the spirit. In the human being, what constitutes biological life also constitutes the person. The person actualizes itself in the body and the body is, therefore, its expression. In the body we may see what is invisible as spirit. Because the body is the person become visible, and the person is an image of God, the body, taken in its full network of relationships, is also the space where the divine becomes imaged, expressed, seen. This is why, from the very beginning, the Bible portrays the mystery of God in images of the body and of the world that is ordered to that body. In doing so, the Bible is not creating external images for God; rather, if it can use corporeal things as images and if it can talk about God in parables, it is because these things truly are images. Thus, by the use of such analogous language the Bible does not alienate the corporeal world but rather names the most real thing about the world, the core of what it is.
This is also the context in which the Bible understands the Incarnation: the assumption, by the biblical Word, of the world man and of the human person that expresses itself in the body; the transformation of that world into a likeness and an image of god through the biblical proclamation – these things are already an anticipated incarnation. The Incarnation of the Logos brings to perfection what had been at work from the beginning in the story of the Bible. At all points the Word is continually drawing to itself, as it were, the flesh of the world, making it into its own flesh, the place of its habitation. We can say, on the one hand, that the Incarnation can occur only because the flesh had always been the expressive form of the spirit and thus a possible dwelling place for the Word; and, on the other hand, we must affirm that in this sense it is only the Incarnation of the Son which definitively confers on man and on the visible world their real significance.
… The Incarnation is not an end in itself; by its very nature it is oriented toward transcendence and, hence, toward the dynamism of the Easter mystery. The Incarnation is founded on the fact that God in his paradoxical love, transcends himself and assumes flesh and thus enters the very passion of being human. But in this self-transcendence of God what really comes to the forefront is, contrariwise, that interior self-transcendence of the whole creation which the Creator had woven into its very fabric: the body is a movement of self-transcendence that tends to spirit, and spirit is a movement of self-transcendence that tends to God.”
The Large Question: Can God Suffer as God? The received theological tradition says, No. And this because suffering implies a diminution of being, and God as infinite Being and Spirit, cannot suffer ontological diminution, and therefore cannot suffer. Thus Thomas Weinandy:
“Within the Incarnation the Son of God never does anything as God. If he did, he would be acting as God in a man. This the Incarnation will never permit. All that Jesus did as the Son of God was done as a man – whether it was eating carrots or raising someone from the dead. He may have raised Lazarus from the dead by his divine power or, better, by the power of the Holy Spirit, but it was, nonetheless, as man that he did so. Similarly, the Son of God did not suffer as God in a man, for to do so would mean that he was not a man. The Son of God suffered as a man.
Nonetheless, if the Son of God suffers as man, why does this suffering not affect his divinity given that the Son of God is equally God? Here we enter the heart of the mystery. While the mystery of the Incarnation, by its very nature, remains, the answer lies in the fact that as God the Son is not deprived of any good which would cause him to suffer as God. If the Son of God, as God, were deprived of some good which would cause him to suffer as God, it would mean… that he is actually no longer God" ("Does God Suffer?" UNDP  205).
The underlying metaphysics of this impassibility of God as God is presented by Weinandy in a recent First Things article:
“Creatures constantly change because they continually actualize their potential either for good, and so become more perfect, or for evil, and so become less perfect. God is not in this act/potency scheme of self-actualization. God, Aquinas argued, is “being itself” or “pure act” and so cannot undergo self-constituting change by which He would become more perfect. Two pertinent points flow from this.
First, by being pure act, God possesses the potential to perform acts that are singular to His being pure act. While we cannot comprehend how God, as pure act, acts, the act of creation is God acting as pure act, whereby created beings are related to God as He is and so come to exist. Thus, the very act of creation that assures the wholly otherness of God is the very same act that assures creation’s immediate, intimate, dynamic, and enduring relationship with God as God truly is in all His transcendent otherness. Second, as pure act or being itself, all that pertains to God’s nature is in pure act. While God and rocks may both be impassible, they are so for polar opposite reasons. A rock is impassible because, being an inert impersonal object; it lacks all that pertains to love. God is impassible because His love is perfectly in act (“God is love”) and no further self-constituting act could make Him more loving. God is absolutely impassible because He is absolutely passionate in His love. Thus creatures, and particularly human beings, through the act of creation are immediately and intimately related to God as He exists in His perfectly actualized love” (Does God Suffer? Thomas G. Weinandy 2001 First Things 117 [November 2001]: 35-41).
On the contrary...
Benedict XVI as Cardinal Ratzinger seems to contradict this assessment of the suffering humanity and impassible divinity in Christ which he claims is rooted “on the one hand [in] a biased rationalistic theology and, on the other, banalization of the Jesus understood as image of God that debases the love of God to a cheap conception of a merely `nice’ God” (“Paschal Mystery…” ibid., 155). The core of Ratzinger’s thought is the following:
“Over against the Stoic ideal of apatheia and the Aristotelian god, who is the very thought of thought, we have the heart as quintessence of the passions, without which passions the Passion of the Son could not have occurred."
He then makes reference to a most significant remark of St. Justin Martyr (+ 165): Passionum nostrarum particeps factus est (“He became a partaker of our passions”) from the encyclical “Haurietis Aquas” of Pius XII.
“Now the Fathers had come from the ethical ideal of the Stoics, the ideal of the passionlessness of the wise man whereby philosophical insight and the will come to dominate and overcome irrational feeling. For the Fathers, the, Christ’s partaking in man’s passions was precisely one of the points where the synthesis of Greek thought and biblical faith was most significant. The God of the Old Testament, who becomes angry or compassionate and who can love, often seemed to be more at home with the gods of the superseded religions than with the exalted concept of God in ancient philosophy – the concept which had made possible the advent of monotheism in the Mediterranean world. From Cicero’s Hortensius Augustine could not find his way back to the Bible, and thus the temptation was always strong to opt for the kind of Gnosticism that divided the God of the Old Testament from the God of the New Covenant. On the other hand, however, it could not be overlooked that the figure of a Jesus who is afraid, who becomes angry, who rejoices and hopes and is despondent, is directly in line with the Old Testament idea of God – indeed, that it is only in Jesus the Logos become man, that all the anthropomorphisms of the Old Testament attain their most extreme radicalization and utmost depth. The Docetistic attempt to make of Jesus’ suffering a mere appearance lay close at hand on the Stoic side. But to every unprejudiced reader of the Bible it must have been clear that this option negated the very heart of the biblical proclamation concerning Christ – the Easter mystery.
“The suffering of Christ, then, was the unshakeable fact; but there is no such thing as a Passion without the passions: suffering presupposes the ability to suffer, the sensibility and it feeling faculty. In the patristic period it was Origen who most profoundly grasped the theme of the suffering God, and who also most straightforwardly declared that this theme cannot be reduced to the suffering humanity of Jesus, but that it colors the Christian conception of God himself (underline mine). The fact that the Father allows the Son to suffer constitutes the Father’s own Passion, and this is also the suffering of the Spirit, of whom Paul says that he sighs in us and that, in us and for us, he bears the passion of our longing for the fullness of redemption (Rom 8, 26f). And it was also Origen, moreover, who formulated the normative hermeneutic on the theme of the suffering God: whenever you hear of God’s passions and sufferings, says Origen, you must always relate these to his love. God is a sufferer only because he is first a lover; the theme of the suffering God follows from the theme of the loving God and continually points to it. The decisive step that the Christian concept of God takes beyond that of the ancients is the realization that God is love."Ratzinger concludes the point decisively: “(Theology today) does confront a technical rationalism which degrades the emotional element in man to the status os the irrational and which likewise assigns to the body the role of mere instrumentality. To this corresponds a certain scorn for the emotional in piety, emotionalism that has frequently remained chaotic and disjointed. We could say that disdain for pathos leads to its becoming pathological, whereas our actual concern should be to integratea pathos into the whole of our human existence and of our presence before the face of God” (Paschal Mystery… ibid. 153-155).
Augustine gives the meaning of the feast of the Sacred Heart: “Let us return to the heart that we may find Him” The pierced Heart of Jesus overturns the Stoic isolationism and individualism of American society if we will let it. Ratzinger says, “This Heart consists, not of self-preservation, but of self-abandonment. It saves the world by opening itself out. The crushing overturn of the opened Heart is the very content of the Easter mystery. The heart does save indeed; but it saves by giving itself away utterly. IN the Heart of Jesus, therefore, we are face to face with the center of Christianity. In this Heart everything is said concerning that truly new overturn that takes place in the New Covenant. This Heart calls out to our heart. It invites us to forsake our vain attempts at self-preservation and to find, in an imitation of his love – in our giving ourselves away to him and with him – that fullness of love which alone is eternity and which alone sustains the world.”
"Look, we have to love God not with our heart only, but with His..." St. Josemaria Escriva, Furrow 809.
Raniero Cantalamessa comments that the heresy which affirmed that God indeed suffered as God had denied the distinction between the Persons of the Father and the Son. Hence, if the Son suffered, so must the Father have suffered.The adversaries of this heresy called the proponents of the theory "Patripassians," that is, those who attribute Passion to the Father. Cantalamessa remarked that "This, however, was a very different idea to the orthodox one according to which the Father, while remaining the Father, participated in the Son's Passions who remained the Son, that is, a distinct person. As usually happens in such cases, the rejection of this heresy brought with a rejection of the truth that preceded it as if to leave the heresy without claim. The theme of the compassion of the Father disappeared from the language and conscience of the Church; it was completely disregarded. It became usual to make a strict distincton between the passion as something `willed,' which is common to the Father and to Son, and the passion `suffered' which belongs to the Son only. The general and inexorable process of adaptation to the culture of the time caused the Biblical idea of God's suffering to be sacrificed to the Greek idea of God's impassibility...
Nevertheless, it has remained a point in the dogma of the Church, from which new departures can be made. The faith of the Church, despite various disputes, has always continued to profess `Theopaschism,' the doctrine of the suffering of God in Christ, holding strongly to the old affirmation that `God suffered.'[Denzinger-Schoenmetzer, 201, 222]. The meaning of this dogmatic affirmation is that God suffered `in the flesh,' but we know from theology that `who' suffered in the flesh - the subject - is the person of the Son, that is, God. One member of the Trinity suffered' and if one suffered, because of the reciprocal interpenetration of the three divine persons the whole Trinity suffered" (Raniero Cantalamessa, "Life in Christ, A spiritual Commentary on the Letter to the Romans," The Liturgical Press  93-94).
Bernard Lonergan exercised the following questions and answers that hinge on the distinction between subject and nature.(St. Thomas affirmed unambiguously, "Actiones sunt suppositorum [subjects exercise actions]:
Q. Who suffered under Pontius Pilate?
A. Jesus Christ, his only Son, our Lord.
Q. Did he himself suffer, or was it somebody else, or was it nobody?
A. He himself suffered.
Q. Did he suffer unconsciously?
A, No, he suffered consciously. To suffer unconsciouisly is not to suffer at all. Surgical operations cause no pain, when the patient is made unconscious by an anesthetic.
Q. What does it mean to say that he suffered consciously?
A. It means that he himself really and truly suffered. He was the one whose soul was sorrowful unto death. He was the one who felt the cutting, pounding scourge. He was the one who endured for three hours the agony of the crucified.
Q. Do you mean that his soul was sorrowful but he himself was not sorrowful?
A. That does not make sense. The Apostles' Creed says explicityly that Jesus Christ, his only Son, our Lord, suffered under Pointius Pilate.
Q. Do you mean that his body was scourged and crucified but he himself felt nothing?
A. No, he felt all of it. Were our bodies scourged and crucified, we would feel it. His was scourged and crucified. He felt it.
Q. Is not Jesus Christ God?
A.. He is.
Q. Do yo mean that God suffered?
A. In Jesus Christ there is one person with two natures. I do not mean that the one person suffered in his divine nature. I do mean that the one person suffered in his human nature.
Q. It was really that divine Person that suffered though not in his divine nature?
A. It was. He suffered. It was not somebody else that suffered. It was not nobody that suffered." (Collected Works of Bernard Lonergan 4 Collection, University of Toronto Press  179-180).