It's a game, and a community. I try to help people whenever I can, even if it doesn't benefit me. I've seen the "you scratch my back and I scratch yours" mentality in groups of people on other servers, and it never ends well. When that mentality is all you operate off of, I think it zaps the fun from the game.
Besides, I have loads of fun getting to know new players and talking to them. Endgame content is fun as well, but it's just as much fun to run through content with people that are brand new and don't know the mechanics. In my experience, they're typically nicer and seem to notice seniority, so they won't argue when you give them advice.
Typically.