What causes burnout?

When I try to figure out how to balance personal happiness with saving the world, I think a lot about burnout. If I make a major change—e.g. changing careers—what are the chances I’ll burn out and become a lot less useful as a result?

I’ve never burned out, so I don’t know where that edge is for me (plus I suspect the edge moves around depending on circumstances). I’m obviously biased on the topic: there’s the temptation to tell myself “This will prevent burnout and make me more effective in the long run” every time I want to do something.

Some things people here have described as causing burnout:

Going through the motions of a religion you don’t believe in

Training yourself to feel guilty whenever you relax

Pursuing altruism too exclusively (That post suggests Bostrom’s parliamentary model as an antidote).

Do you have more examples? Have you burned out? Are there things that you think have kept you from burning out?