A quote from Mere Chrisianity by C.S. Lewis

The Christian is in a different passion from other people who are trying to do good. They hope, by being good , to please God if there is one; or – if they think there is not – at least they hope to deserve approval from good men. But the Christian thinks and good he does comes from the Christ – inside him. He does not think God will love us because we are good, but that God will make us good because He love us; just as the roof of the greenhouse does not attract the sun because it is bright, but becomes bright because the sun shines on it.