Throughout history there have been a lot of theories to explain why people do what they do. Elder Dallin Oaks gave an interesting perspective on these reasons (entire talk here) in an LDS setting. He covers from the most selfish of reasons to what he explains as the best reasons to do what we do.
For example, at the selfish end of the spectrum, hedonism suggests that we do everything we do to avoid pain and increase pleasure. In other words, human beings are entirely selfish. According to this theory, I would never do the dishes unless it gave me pleasure (e.g., verbal praise from my wife) or helped me avoid pain (e.g., having her mad at me). All of human actions could be explained with this theory. According to a hedonist, Christ did not suffer on the cross to save mankind, but so that he could be worshiped forever and avoid being seen as a failure or coward.
Of course, hedonism is not a theory most people agree with.
According to Oaks, one of the greatest reasons for anyone to do something is for the hope of an eternal reward, such as eternal happiness or salvation. I disagree with Oaks on his interpretation of the rightness of such motivation. I really don't see that reason for action as different from hedonism.
For example, some readers of this blog have attempted to divert my course of action by telling me that my family and I will lose eternal rewards because of my choice. In other words, to them it does not matter so much whether the action is right or wrong, but what rewards will be gained or lost from the decision.
Oaks suggests that the greatest reason to do anything is out of charity, or "the pure love of Christ". I like his conclusion very much in that love is a far better reason to act than selfishness.
I would like, however, to further suggest that an even higher reason to act is for the simple reason that it is the right thing to do. If you meet a man without a coat on a freezing day, give him your coat not because you want to feel better about yourself (hedonism). Give him your coat not because God expects it of you (eternal reward/hedonism). But give him your coat because he is cold - because he needs a coat.
Of course, regardless of why you give him the coat, give him the coat. If you do it to impress that cute girl across the street, at least he's getting the coat. But may I suggest that to do it because it's the right thing to do is the best reason to do anything?
Regarding my decision, some have indicated that it must be out of selfishness; I must have left the Church so that I could free up some time on Sundays or so that I could start gambling or drinking. Contrary to what such individuals may think, this has been a very painful process for me. I have never been so attacked in all my life - not even from the German people on my mission for the LDS Church. I have never felt so judged by people I love and respect.
Yet I have chosen this course because it is right. I have acted because I know it is the right thing to do. I did not do this because I would get a great reward. I did not do this out of love for anyone or anything (although I love my daughter more than anything and believe it will be for her benefit). I did this because it is the right thing to do. I don't expect all the readers to understand or want to understand that. At some point it doesn't matter if they understand. It is still right.