[hexagonc]

I think it is easy to come up with examples of people doing purely altruistic things. One simple example would be an atheist that sacrifices himself for his country. He knows there is no reward at the end of the tunnel and may not even particularly want to do it but maybe feels it is his duty for the "greater good". Purely altruistic behavior can be justified by anyone who is fully committed to, say, utilitarianism and I'm sure there are other moral philosophies that can justify it without any consideration for personal reward.

I think you have to distinguish the case between someone doing a kind deed and feeling good versus someone doing a kind deed because they expect to feel good. Feeling good because you did something good is involuntary and for most people unavoidable. It can be a side-effect of the deed, rather than the cause.