One might ask, are people really moral if they are told that they will be rewarded in heaven for what they do? Wouldn't that be like offering someone $50 to be kind to their annoying little sister for a week?
In the $50 case, the reward is extrinsic to the deed, or the character necessary to do the deed. One idea that, for example, comes out of C. S. Lewis, is that if we are going to live forever, then unless our characters are properly formed, they will cause more and more harm to ourselves and others. The only way to be the kind of person who can enjoy eternity is someone allows God to create in him a Christlike character. Sex, drugs, and rock and roll will become boring or worse after a million years. So on this view, heaven isn't just a cash reward, it becomes possible for a person because they open themselves up to character transformation. Morally good acts reflect the kind of character that will make a heavenly life possible for a person.
HT: Steven Clauer