It can either be a sign of submission - like the puppy to his mother or on the contrary a sign of dominance, if done excessively and obsessively like bitch would groom her pups...
If you ask the dog to stop and he stops then it is merely healthy submission / affection if you want!
Some dogs may also do it along with the excitement of seeing someone again or someone new, as they usually try to smell the face of the person (to greet them, "get to know them"), it can turn into licking!