The problem might be: I associate my drive to change the world for the better with the poor condition of my own life. Not in all ways, of course--I would be fine with having a blockbuster academic career filled out with various sorts of social activism--as long as I wasn't happy.
I value my drive to change the world for the better. There's something terrible about the norm of acceptance; accept the genocides, the lies, the general unpardonable suffering of other human beings. Accept because they aren't here, and potential solutions are complicated. It's true that there's no social pressure to say these things are alright, but to be normal is to do nothing, or to do only what is comfortable--and, to condemn the norm is called unreasonable.
It doesn't seem like it would be possible to be happy without cutting yourself off from the incredible amount of pain that goes on in the world. It seems like you'd have to stop seeing all those people, who constantly hurt, as people. I'm afraid of being the norm; I feel that when I put resources into things that make me happy, they could be going to something better. I feel that when I'm happy I'm complacent. I feel that when I'm happy I'll start being part of the problem instead of part of the solution.