Stress or boredom. I've gotten better so that now, I typically do not bite when I am bored. But, if I get really really stressed about something or start to feel helpless about a situation and am running the problem over and over again in my head, I will find myself biting my nails. It is weird because now I typically don't remember when I start doing it, I just kind of come to in the middle of it.

I find I am prompted by scenarios- like filling out paper work. I assume I started nail bitting during tests, like bothgodown describes, a 'stressful' situation. But now I even bite my nails when Im not stressed but am confronted by a similar scenario.