Well.
I joined because I'm curious and opinionated... which is the reason I join most forums, I suppose. I tend to be very shy in real life, but on forums, I tend to post a lot.
I've never "stayed" at any forum, though I'm hoping I'll stay at the writing forum for at least the next two years, since to do otherwise would mean I'd given up on a goal. The main reason I don't stay with a forum is that I have a lot of different interests, but tend to be rather singleminded about them. Right now, although I wasn't intending it to be the case, BDSM is the one I'm currently somewhat fixated on. I've been (barely) managing to keep up with the writing one, but I come here more often. The point is, though, that usually, I can only stay at one forum at a time, and I tend to post a lot on that forum when I'm there. Then, something will happen, and I'll either get interested in another topic and try to do the two or more forums thing or I'll get behind and feel I *need* to catch up (which is silly, I realize), but not want to do so, and then end up finding a new forum.
In any case, it's (so far) only been in one case that the actual forum was bad. I tend to be good at picking forumsIncidentally, that forum was mainly devoted to people much younger than myself (i.e. teens) and there were a lot of difficult-to-read posts and a lot of stupid contests. Most of the time, I move on to a new forum because the new topic is more exciting than the old one. And I have been known to come back to old forums when the topic is interesting again.