What God Desires: A Pure Heart

Have you ever wondered why it is that Abel's sacrifice was accepted by God, yet Cain's was not? Or why Saul was rejected by God as the king of Israel for offering the sacrifice before Samuel, yet David was not after the whole incident with Bathsheba and the issue with his son Absalom that caused a divide in the nation? I think what is going on here is less of God playing favorites, but rather something else dealing with these two men. Let me propose that the sacrifice and worship which God accepts is that which comes from a pure heart.