My #1 reason is that I don't have enough to do.

I've complained about it so often to colleagues and supervisor it has become a running joke, but nothing changed. Previous job was even worse. I've been doing 2-3 weeks of work, followed by 1-2 months (if not more) of just sitting there, even when I have 2 or 3 projects in parallel. Somehow everyone else seems to be busy all the time.