One of the biggest reasons people are motivated to do something they consider good is the 'good feeling' it brings them. This is disguised in many ways. One is righteousness, others are less subtle. Let me show you two examples:
1) Being praised after death (like this scene)

This is the case when you know that your life is short and you will die for a venture. So you imagine your glory (praise among people) after death. Your sacrifice may or may not work out as planned though.

2) Being praised before and after death. Take James Joyce's example. When he wrote Ulysses, he said that he had put so many knots in it that it would take eternity for people to decipher them all.

This is why I sometimes wonder whether selflessness exists at all. No matter what personal loss you go through to achieve something you consider right, there is that little 'good feeling' that rewards you. To say people do things solely for merit, is I believe untrue.

The 'good feeling' might not be the only driving force, but it certainly matters in your decision at a subconscious level. That is why people turn evil I believe, they take their whims for right and wrong.