According to Dr. Patty Khuly of Vetstreet, the main reason why dogs lick people is to show affection. However, there can be many other reasons for licking including natural instinct, sensory exploration, attention seeking and playfulness.
Although licking is part of a dog's nature and is usually a positive behavior, Dr. Khuly warns against extreme licking, which occurs when a dog's licking behavior becomes excessive. Dogs who are affected by obsessive-compulsive disorders may experience the condition by excessively licking people, objects or themselves. If a dog displays extreme licking behavior, a veterinarian may be able to treat the dog's obsessive-compulsive condition and stop the behavior.