Imagine that you got no satisfaction at all from bringing pleasure to others, but you did it anyway. What would be the reason?

The response to that would be that you only do things that give others pleasure because the feeling of helping others is pleasurable to you or because you expect something in return, and that if neither of those were the case, you wouldn't do it. (I don't necessarily agree with that — I'm pretty sure I don't — but I do believe that's how they'd reduce it.)