The human heart is heavy and hardened. God must give man a new heart. Conversion is first of all a work of the grace of God who makes our hearts return to him: "Restore us to thyself, O LORD, that we may be restored!" God gives us the strength to begin anew. It is in discovering the greatness of God's love that our heart is shaken by the horror and weight of sin and begins to fear offending God by sin and being separated from him. The human heart is converted by looking upon him whom our sins have pierced:
Let us fix our eyes on Christ's blood and understand how precious it is to his Father, for, poured out for our salvation it has brought to the whole world the grace of repentance.
Source: Catechism of the Catholic Church (Vatican.va)