The logic and arithmetic of God's mercy is not so easily discernible to us as we so often like to believe. The Gospel today reminds us that God is Himself the Prodigal Employer, who gives to each out of his own generosity. We are meant to rejoice and to take delight when sinners return to the Lord and receive the reward of Divine Life. But instead often we can become envious because God is generous.  
God's mercy comes to us powerfully when we recognize our need for it. And we all need it. Let us rejoice whenever someone comes running to the Lord.
For the daily readings, see the USCCB.