Probably not related to licking their humans!
, but just between cats, I think grooming can be a sign of dominance, as well as a sign of loving contentedness among "family" members. Our girl cat Geisha is always trying to groom Kuri, the new baby boy, but he usually doesn't like it and tries to get away from her, although sometimes he gives in and just sits there and lets her groom his face (like when he's "given up").
My friend's cats are also like this, even though they are littermates. The sister grooms the brother, and she is also a few pounds bigger than him. He lets her, but sometimes he gets upset and tries to turn his head and walk away so she can't groom him. Another friend's mother cat grooms the daughter cat, but never vice versa.