The parables of lost things in Luke 15 show us God's heart for lost souls.  The parables of the lost sheep and lost coin point to normal human behavior to show that it is reasonable for God to seek lost souls because they are important to him.  The Prodigal Son, or more correctly, the lost son shows that God eagerly welcomes the repentance of even the "worst" sinners.  He graciously runs to meet the repentant person to cover them in the robe of Jesus blood so that they can enter into his eternal celebration.  The older son had sin issues of his own but refused to accept his father's gracious response and enter into the celebration.  As children of God, we should share his love for the lost.