Nail-biting syndrome is the excessive habit of biting your nails, which distorts the shape of the nails.
There are many people with a nail-biting habit. The reason varies for each person. It could be due to stress, or just a meaningless habit developed over childhood. Either way it's not a good habit.
We are more inclined to take care of things we put more effort into. The more time spent on it, the more we are attached to it. This is natural human behaviour.