Some people bite the insides of their cheeks due to a condition known as keratosis, a type of obsessive-compulsive disorder, according to GoodTherapy.org. The repetitive behavior often begins in childhood and can last into adulthood. It may be a compulsive reaction to anxiety and stress.
Another condition that may cause chewing of the inside of the cheeks out of compulsion is dermatophagia, according to Wikipedia. Sufferers additionally may chew on the skin around their joints and fingernails along with the inside of their mouths and lips, resulting in blisters. If the behavior continues, calluses may form where the biting occurs, the site notes.