S.E.
For me it is anxiety driven. I can go weeks with out bothering, but put me in a situation where I'm uncomfortable, being able to silently pick my nails (behind my back, in a pocket, away from others, etc.) and it helps me stay in the situation, rather than running.
I also do it if I am totally bored.
My mother tried to break me of that habit, but it didn't work. Maybe pretty polish will help. I know my daughter started and I gave her a goal of 1 wk with no biting or picking and she could go to the nail salon w/me. She loved it. Now she picks off the polish, which I suspect is better than picking or biting her nails.
Stephanie