An argument can be made that all actions people take are for their own self-interest. Even things like helping the community is done for reciprocal benefits for yourself plus the general increase in respect.
Another point is that if all people just act for their own self-interest, they will have a better life themselves, and if everyone does the same, some sort of proper balance will be achieved.
I have been having some occasional discussions with my friends over this. I personally disagree, but would love to hear what others have to say. Feel free to discuss!
Our decisions are heavily influenced by emotion. We have the sense of empathy, which is an adaptation that makes communal living work. Empathy motivates us to do things for other people sometimes. You can say, “you do helpful things to satisfy your own emotional needs.” But that’s pretty much saying, “you do helpful things because you want to.” I think self-interest is a big factor in how we act, but I don’t think it’s the only factor.
Some people completely lack any empathy at all.