In today's Gospel reading from Matthew 12 (and in Matthew 9 too), Jesus quoted "I desire mercy, not sacrifice" from Hosea in response to the Pharisees' question. I think I see why God prefers mercy over sacrifice from us. Let's say that you have two sons trying to please you. The first son knows that he is unworthy of your love and does things to "punish" himself in some way, hoping to win you back, while the second one, also feeling unworthy but goes on joyfully doing good deeds to help others. I think you would be more pleased by the second son and even feel sorry for the first one, for self-sacrifice is still self-centered – an act with no joy in it – while helping your neighbor is following the second greatest commandment of God.