What Causes The Habit Of Lip Biting?

i have a strange habbit of biting/rubbing my lips specially my lower lip due to this my lower is now bigger/thicker....its now pink in color... most distinguished feature of my face is my lower lip..when i was a child i used rub my lower lip with a finger....even now i cannot resist doing it...i used chapsticks which had helped me to some extent but not completely but later i realised it should be a kind of Nervous problem..pls help