"The symbol and express image of the infinite love of Jesus Christ . . ."

This is from Leo XIII on the Sacred Heart. Pius XII offers this quote in his marvelous encyclical Haurietis Aquas.

You should really read the whole thing, so here are some tidbits to pique your interest:

Moreover there are those who consider a devotion of this kind as primarily demanding penance, expiation and the other virtues which they call "passive," meaning thereby that they produce no external results. Hence they do not think it suitable to re-enkindle the spirit of piety in modern times. Rather, this should aim at open and vigorous action, at the triumph of the Catholic faith, at a strong defense of Christian morals. Christian morality today, as everyone knows, is easily contaminated by the sophistries of those who are indifferent to any form of religion, and who, discarding all distinctions between truth and falsehood, whether in thought or in practice, accept even the most ignoble corruptions of materialistic atheism, or as they call it, secularism.

Since our divine Redeemer as our lawful and perfect Mediator, out of His ardent love for us, restored complete harmony between the duties and obligations of the human race and the rights of God, He is therefore responsible for the existence of that wonderful reconciliation of divine justice and divine mercy which constitutes the sublime mystery of our salvation. On this point the Angelic Doctor wisely comments: "That man should be delivered by Christ's Passion was in keeping with both His mercy and His justice. With His justice, because by His Passion Christ made satisfaction for the sins of the human race, and so man was set free by Christ's justice; and with His mercy, for since man of himself could not satisfy for the sin of all human nature, God gave him His Son to satisfy for him. And this came of a more copious mercy than if he had forgiven sins without satisfaction: Hence St. Paul says: 'God, who is rich in mercy, by reason of His very great love wherewith He has loved us even when we were dead by reason of our sins, brought us to life together with Christ.'"

But St. Augustine, in a special manner, notices the connections that exist between the sentiments of the Incarnate Word and their purpose, man's redemption. "These affections of human infirmity, even as the human body itself and death, the Lord Jesus put on not out of necessity, but freely out of compassion so that He might transform in Himself His Body, which is the Church of which He deigned to be the Head, that is, His members who are among the faithful and the saints, so that if any of them in the trials of this life should be saddened and afflicted they should not therefore think that they are deprived of His grace. Nor should they consider this sorrow a sin, but a sign of human weakness. Like a choir singing in harmony with the note that has been sounded, so should His Body learn from its Head."

Thus, from something corporeal such as the Heart of Jesus Christ with its natural meaning, it is both lawful and fitting for us, supported by Christian faith, to mount not only to its love as perceived by the senses but also higher, to a consideration and adoration of the infused heavenly love; and finally, by a movement of the soul at once sweet and sublime, to reflection on, and adoration of, the divine love of the Word Incarnate. We do so since, in accordance with the faith by which we believe that both natures -- the human and the divine -- are united in the Person of Christ, we can grasp in our minds those most intimate ties which unite the love of feeling of the physical Heart of Jesus with that twofold spiritual love, namely, the human and the divine love. For these loves must be spoken of not only as existing side by side in the adorable Person of the divine Redeemer but also as being linked together by a natural bond insofar as the human love, including that of the feelings, is subject to the divine and, in due proportion, provides us with an image of the latter. We do not pretend, however, that we must contemplate and adore in the Heart of Jesus what is called the formal image, that is to say, the perfect and absolute symbol of His divine love, for no created image is capable of adequately expressing the essence of this love. But a Christian in paying honor along with the Church to the Heart of Jesus is adoring the symbol and, as it were, the visible sign of the divine charity which went so far as to love intensely, through the Heart of the Word made Flesh, the human race stained with so many sins.

And so we can easily understand that the devotion to the Sacred Heart of Jesus, of its very nature, is a worship of the love with which God, through Jesus, loved us, and at the same time, an exercise of our own love by which we are related to God and to other men. Or to express it in another way, devotion of this kind is directed towards the love of God for us in order to adore it, give thanks for it, and live so as to imitate it; it has this in view, as the end to be attained, that we bring that love by which we are bound to God to the rest of men to perfect fulfillment by carrying out daily more eagerly the new commandment which the divine Master gave to His Apostles as a sacred legacy when He said: "A new commandment I give to you, that you love one another as I have loved you. . .This is My commandment that you love one another as I have loved you." And this commandment is really new and Christ's own, for as Aquinas says, "It is, in brief, the difference between the New and the Old Testament, for as Jeremias says, 'I will make a new covenant with the house of Israel.' But that commandment which in the Old Testament was based on fear and reverential love was referring to the New Testament; hence, this commandment was in the old Law not really belonging to it, but as a preparation for the new Law."