There is no such thing as pure altruism, where did I say there was? (https://en.wikipedia.org/wiki/Altruism#Evolutionary_explanat...) I gave my own reasons, that's all. But it's not monetary.
Why do people like you seem determined that nothing good can exist in other people. Your tribe turns everything to shit.
There you go, so you help others to feel happiness yourself. In which case, its not purely altruistic.