This pervasive habit is sometimes a sign of an underlying mental health condition, such as anxiety or an impulse control disorder. It isn't likely to cause long-term damage or interfere with fingernail growth as long as the nail bed remains intact, but it can lead to infection. As you chew on your fingertips, bacteria or possibly even pinworm eggs under the surface of the nails can enter your mouth. When you have a sore on your lips or in your mouth, it's even easier for germs to get in.
If I didn't get you with "pinworm," go to your pharmacy and pick up one of the bitter-tasting clear polishes designed to discourage nail biters from biting. Research shows they really can work.