A desire to help the community, I guess.
I dunno, I'm a mod on two different comic book forums. I was asked both times out of the blue and I've had different experiences with each. With each forum, it's been great to help use the resources I have to help the site grow and I've made a bunch of nifty contacts and good friends along the way.
However, I wouldn't recommend it to anyone. It's a thankless job and some of the crap I've had to deal with has been absurd. The internet seems to bring the worst out of people and between the butthurt drama here and some of the more creative trolls I've encountered because of the other site, it really kills a lot of my desire to be on the Internet at all sometimes.
Thanks for the honest answer. I still don't understand it. I guess it's the same reason as certain people want to be cops. I would never want to be a police officer, but certain people do.