Lack of forgiveness blocks the grace of God in our lives.  It brings death--spiritually, emotionally, and often, physically.  It is the very opposite of what Jesus desires for us.  Our will--freely choosing to hold on to anger, bitterness, resentment and hostility--becomes an impenetrable fortress preventing God's life from entering our heart, which, through the privation of grace, becomes stony, hard, and cold.  This interior reality eventually affects all aspects of our being.