As I have written here before, the heart is the symbol of the center of the human person. It is a theological term which points first of all to God and to God's activity deep within us. It is not so much that we have a heart and then God comes to dwell there; it is that where [and to the extent] God dwells within us and bears witness to himself, we have a heart. The human heart (not the cardiac muscle but the center of our personhood the Scriptures call heart) is a dialogical event where God speaks, calls, breathes, and sings us into existence and where, in one way and degree or another, we respond to become the people we are [and are called to be]. It is therefore important that our hearts be open and flexible, that they be obedient to the Voice and love of God, and so that they be responsive in all the ways they are summoned to be.
Bearing this in mind it is no surprise that the Scriptures speak in many places about the very worst thing which could befall a human being and her spiritual life. We hear it in the following line from Ezekiel: [[If today you hear [God's] voice, harden not your hearts.]] Many things contribute to such a reaction. We know that love is risky and that it always hurts. Sometimes this hurt is akin to the mystical experience of being pierced by God's love and is a wonderful but difficult experience. Sometimes it is the pain of compassion or empathy or grief. These are often bittersweet experiences, but they are also life-giving. Other times love wounds us in less fruitful ways: we are betrayed by friends or family, we reach out to another in love and are rejected, and a billion smaller losses wound us in ways from which we cannot seem to recover.
In such cases our hearts are not only wounded but become scarred, indurated, less sensitive to pain (or pleasure), stiff, and relatively inflexible. They, quite literally, become "hardened" and we may be fearful and unwilling or even unable to risk further injury. When the Scriptures speak of the "hardening" of our hearts they use the very words medicine uses to speak of the result of serious and prolonged wounding: induration, sclerosis, becoming calloused. Such hardening is self-protective but it also locks us into a world that makes us less capable of responding to love with all of its demands and riskiness. It makes us incapable of suffering well (patiently, fruitfully), or of real selflessness, generosity, or compassion.
It is here that the symbol of the Sacred Heart of Jesus' is instructive and where contemporary devotion to the Sacred Heart can assist us. The Sacred Heart is clearly the place where human and divine are united in a unique way. While we are not called to Daughterhood or to Sonship in the exact same sense of Jesus' (he is only "begotten" Son, we are adopted Sons --- and I use only Sons here because of the prophetic, countercultural sense that term had for women in the early Church along with its derivative nature --- whether male or female we are sharers in Jesus' own Sonship --- we are meant to be expressions of a similar unity and heritage; we are meant to have God as the well spring of life and love at the center of our existence.