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?
http://www.psychologytoday.com/blog/real-healing/201107/stressed-out-maxed-out
"Burnout can have its roots in childhood. Studies show that children who were bullied, who experienced abuse or neglect, had a parent in prison, lived in a home where there was mental illness, violence, substance abuse or the loss of a parent through any cause were more prone to develop diseases such as emphysema, mental health problems, substance abuse and obesity. These childhood experiences can predispose to burnout and the associated health and psychological problems in adolescence and adulthood."
Not everyone has a traumatic childhood, but if you do or at least have something you need to get to the bottom of with yourself - do so. Problems probably rear their ugly heads in more ways than just burnout, so get to ridding yourself of them.