Why Be Moral?

Heard an interview with theologian Bart Ehrman the other day in which he was saying that because Jesus was an apocalypticist who believed that the world as we know it would end in the very near future and the Kingdom of God would arrive on Earth in a time frame such that some of those listening to him speak would be present, that the motivation he provided for ethical action was egoistic. God would be choosing his kickball team very soon and trust me you want to get picked, so you better do it. One who does not believe in the continuing existence of society would not appeal to a social contract or making the world the best possible place for those in it, since the world and hence the society were not long for being. The only real motivator left is selfishness.

But the world and human society as we know it will continue to be. So the question then stands, why be moral? It is one thing to know what you should do, but it is another to do it. On what basis then should we do what we know we should do? Is it to make ourselves the best human we can be? Is it out of duty? Is it to avoid punishment or social derision? Is it to create a better world for everyone? Is it just to make ourselves feel better? Why be moral?