Some habit is hard to break - SSA David Rossi
We all are human being. We are also the creature of habit.
Why are human beings creature of habit?
because we have the brain capacity to understand and repeat the things that make us feel good/safe/happy etc. things as little as chewing your nails when you are nervous are stored in your brain so when the time comes and you get nervous, the only calming action your brain knows is to start biting. its basically the same for other habits too