Cats groom each other. If you have many cats, you've undoubtedly seen one lick the other's face in a sunbeam.
When you're sad, is your cat more attentive? Dr. Ochoa believes your cat may lick you if you're unwell or anxious, "much as it would another cat in the wild."
Anxious cats might over-groom because they spend so much time grooming. Ziskin believes some cats lick to relieve tension.
Many cats consider their people (you!) their domain.
Kittens bite themselves to clean their hair, and they may bite you during grooming. Little nips convey devotion.
If it's not unpleasant, let your cat lick you. It helps strengthen your pet's trust.