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One possibility is that it is a learned behavior that evolved from kiss feeding, the process by which mothers in some cultures feed their babies by passing masticated food from mouth-to-mouth. However, there are some contemporary indigenous cultures which still practice kiss feeding, but not social or erotic kissing.
Many people do bite or chew on the insides of the lower lip or cheek, perhaps out of boredom or nerves. This habit is often initially prompted by a misdirection of the teeth that causes the person to mistakenly bite into the lower lip while chewing.