Why do people bite their nails even when they do not want to? Why do people bite their nails? People seem to be doing it primarily while contemplating something, or during problem-solving tasks. They seem to understand it is a bad habit, and sometimes they seem to try to active avoid it, but keep on doing it nonetheless so it seems. Why does this happen?
Nail-biting is considered a parafunctional habit. A parafunctional habit is the habitual exercise of a body part in a way that isn't the way that that part was intended to be used. Other common parafunctional habits include bruxism (grinding of teeth), hair-pulling, and picking at skin.

Nobody knows what causes these habits. Possible physical or psychological causes may include emotions (aggression, anxiety, stress, tension), hyperactivity, complications from other disorders (such as obsessive-compulsive disorder), or as a coping or focus mechanism, similar to how babies and young children find it soothing to suck on their thumbs.