Why Does One Have The Habit Of Biting The Lips?

I tend to bit my lip a lot. It's on the inside of my mouth though.. I bite the top lip in my middle the most.. Then when that's too raw to bite I move on to the bottom lip! I bite the right side more than the left. I have special spot I bit they aren't random. I'm not a nervous person or get stressed out! So I don't know what's wrong with me! I have done this ever since I can remember.. Help please?!