My friend has this habit of chewing skin off from his lips and tounge and ect. in his mouth. Why? How?

He chews off skin from his tounge, lips, sides of his mouth... EVERYWHERE. So I'm wondering HOW he does it. It's really painful to me, and many others. And WHY. Thanks!My friend has this habit of chewing skin off from his lips and tounge and ect. in his mouth. Why? How?
Stress and/or nervous habit. It is typically not a type of self mutilation. A friend of mine does this when he gets stressed, it has become a habit. He has done it for so long he isn't even aware he is doing it. When my friend he is distracted he stops doing it. Because he may not be aware he is doing it, Ask himMy friend has this habit of chewing skin off from his lips and tounge and ect. in his mouth. Why? How?
Habit, its like when people pick at their nails, chew their hair.....etc. Its can also be a very mild case of self harming but in habit form. When doesnt he do it most? If he does it when he is stressed it could be a comfort for him. How long has being doing it for? If he has done this from a child it properly hasnt grown out of it. More to the point just ask him, he peoperly doesnt know why himself!