Having been an unpaid moderator for multiple sites, I think I can give a little insight into this question.
1. You take up a hobby and that hobby ends up requiring running a message board. Ultimately, you do it because you enjoy the hobby, not because you enjoy moderating.
2. You spend a lot of time somewhere and get a reputation as being helpful. Someone asks you to assist and you do it because you feel honored that someone asked and don't want to let that person down.
3. You offer to help because you think a place needs cleaning up (which might be true). This is probably the stupidest reason of all!
I've been in all three situations.
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