> I think people doing altruistic things do enjoy them, but they don't do them because they enjoy them. They do them because they need to be done, and the enjoyment is a side effect.

What is the difference between the two conditions? The only one I can think of is that removing the side effect does not change the behavior, but removing the main motivator does. How is this significant though? Is it easier to lose enjoyment from altruistic activities than to lose a sense of responsibility? Is pleasure more fickle than duty?

That distinction, IMHO, also makes the definition of altruism so narrow as to be useless.