From EWTN -
Devotion to the Sacred Heart, as we know it, began about the year 1672. On repeated occasions, Jesus appeared to Saint Margaret Mary Alacoque, a Visitation nun, in France, and during these apparitions He explained to her the devotion to His Sacred Heart as He wanted people to practice it. he asked to be honored in the symbol of His Heart of flesh; he asked for acts of reparation, for frequent Communion, Communion on First Friday of the month, and the keeping of the Holy Hour.
When the Catholic Church approved the devotion to the Sacred Heart of Jesus, she did not base her action only on the visions of Saint Margaret Mary. The Church approved the devotion on its own merits. There is only one Person in Jesus, and that Person was at the same time God and Man. His Heart, too, is Divine -- it is the Heart of God.
There are two things that must always be found together in the devotion to the Sacred Heart: Christ's Heart of flesh and Christ's love for us...
This love of Christ for us was the moving force of all he did and suffered for us -- in Nazareth, on the Cross, in giving Himself in the Blessed Sacrament, in His teaching and healing, in His praying and working. When we speak of the Sacred Heart, we mean Jesus showing us His Heart, Jesus all love for us and all lovable.
Jesus Christ is the incarnation of God's infinite love. The Human Nature which the Son of God took upon Himself was filled with love and kindness that has never found an equal. He is the perfect model of love of God and neighbor.
Every day of His life was filled with repeated proofs of "Christ's love that surpasses all knowledge" (Eph 3:19). Jesus handed down for all time the fundamental feature of His character: "Take My yoke upon your shoulders and learn from Me, for I am meek and humble of Heart" (Mt 11:29). He invited all, refusing none, surprising friends and rivals by His unconditional generosity.
The meaning of love in the life of Jesus was especially evident in His sufferings. Out of love for His Father He willed to undergo the death of the Cross. "The world must know that I love the Father and do just as the Father has commanded Me" (Jn 14:31).
The love that Jesus bore toward us also urged Him to undergo the death of the Cross. At the Last Supper, He said, "There is no greater love than to lay down one's life for one's friends" (Jn 15:13)
The Heart of Jesus never ceases to love us in heaven. He sanctifies us through the Sacraments. These are inexhaustible fountains of grace and holiness which have their source in the boundless ocean of the Sacred Heart of Jesus.
Pax et bonum