I got this while googling for "why do dogs lick your face". Hope this helps:
From the time dogs are puppies they seem to want nothing more than to lick your face. Why do dogs lick your face? Why do they seem to desire this apparent disgusting display of affection? The answer is quite simple, dogs lick your face because this is their natural way of showing that you are their parent/care giver. You see in the wild dogs lick each others faces not only while they are pups but also into adulthood. This action has many different purposes. As puppies the young dogs lick their parents faces to stimulate regurgitation. Young dogs cannot digest meat in its crude form so the parents and uncles and aunts of the pack swallow the meat which then begins to digest in their stomachs. When the adult dogs reach the den the younger dogs begin to lick their mouths which stimulates the regurgitation of the partially digested meat. The puppies then eat this meat.
Dogs also lick faces as a sign of respect. In the wild the dog pack has a definite hierarchy which must be followed at all times and one of the ways in which the dogs show respect to the dominant male and female is by face licking. Woe be to the dog who doesn't lick the face of the Alpha male.
While a dog licking your face may seem like an unsanitary or perverted habit it really is neither. You have understand that dog saliva is full of a antibiotics which help them to stay clean and also protect them from the bacteria etc… which is found on meats. Nor is the desire of a dog to lick your face something with a sexual overture. It is as much a sexual action as when a baby wants to nurse on its mothers milk. Some people find nothing wrong with their dogs desire to lick their face while others find it terribly disgusting. In the end it probably boils down to your personal preference. Dogs should definitely be discouraged from lick human baby or toddlers faces. As in general it can expose the young human to germs which it doesn't naturally encounter. Or for which it might not have sufficient defenses to protect itself.
Can you allow your dog to lick your face? Only you can answer this question since it is your dog, but if you have health concerns then you probably want to teach your dog from the beginning that this is unacceptable behavior.