Nail biting is considered a nervous habit, regarded as a relatively unconscious behaviour that is repeated out of anxiety, stress or boredom. Some people bite their nails so badly that their fingers bleed or the nails are so stunted that there is almost no nail left. 

Relationships

Jess and Jeff began to feel misunderstood, and attacked for who they were, how they behaved, and for what they each wanted or needed. Research shows that when one partner's request for change in the other becomes criticism, the other partner is likely to become defensive.

Anxiety

Have you ever felt an uncomfortable feeling of fear or impending disaster?

This may be your natural response to pressure when facing a challenging and possibly dangerous situation.  This pressure may not be about what’s happening around you, it may be about the demands you place on yourself.