Face-licking is a common behaviour for most dogs - less common - but no less revolting in Humans. Is it sanitary? No of course not! There's no telling where that tongue has been recently. So why do they do it?
Wolves - a creature from which all dog breeds have descended - feed their pups with regurgitated snacks. When the mother returns from the hunt, the pups lick her face and she throws up some tasty titbits. This explains dog's love of eating vomit. So when your beloved pooch licks your face, they really are trying to make you sick. They may be legitimately trying to tell you they are hungry, but of course if you feed them at that point it is simply reinforcing the behaviour.