I seem to go in phases, although my default status is something of a semi-lurk. Sometimes I'm more active, but I'm very much inclined to go into full lurk.
Occasionally a lurking phase will be triggered by, or at least coincide with, the rise of a 'personality' who makes the forum generally a less pleasant place to be. More often it'll be triggered by some stupid trend that gets on my nerves - the zombie obsession was tedious, the 'last ___ you ____ is now ____' thing (y'know, "the last enemy you killed in a game is now dating your sister" or whatever) that's been going on for the past month or two is teeth-grindingly irritating, the problems in the Publisher's Club forum... all that sort of stuff. I find these forum irritants are generally best ignored, disappearing until it stops is a pretty effective solution.
Of course, often I'll read through a topic with every intention of replying, only to find that one or more people have already said exactly what I wanted to say. No point responding if it just amounts to "I agree" and brings nothing else to the discussion. The other thing I do regularly is write out a good, worthwhile response but delete it before posting. Usually this happens when it's a bit of a rant about something - often it's cathartic enough just to write it, by which point the moment is gone and there's no real need to post it out there so I just abandon it.
Probably also worth noting that I was using the site for a full year before I ever bothered to sign up an account.