A cat licks someone to accept the person as a friend. Cats also lick each other as part of a social ritual of cleansing and befriending.
Mother cats clean their kittens as soon as they are born and anytime the kitten has been away from the nest. This is as much an act of loving and caring as it is a practice of cleansing and refreshing. As kittens grow, they mimic this behavior with new friends. Cats enjoy sharing their scents in the process, which binds their friends together with them as part of the same social circle.
Sudden changes in the licking behavior of a cat may be cause for concern. If an older cat suddenly begins to lick or suck aggressively or any cat vigorously licks and bites himself, a check up with a veterinarian is in order.