I became a leftist partially out of mutually-experienced suffering with future comrades, but primarily because I wanted others to be better off and happier. Maybe it was because of happy chemicals from helping others if some reductionist wants to be smug like that, but I still wanted to help others.
I was stating that my own "youth leftist thing" wasn't necessarily self-interest unless you want to boil down the desire to help others and see them be well as neurochemically-driven "selfishness" or whatever.