Licking human faces is part of a dog's behavioral repertoire that often falls under the "cute things dogs do" category. Affectionately referred to as "doggy kisses," facial licks are an instinctive behavior reminiscent of early puppyhood when pups were in the litter with their mom and litter mates. The reasons behind those sloppy slurps vary based on context, and as with other dog behaviors, this behavior can be curbed should it get a tad bit out of hand.
A Learned Behavior
When puppies are born, mother dog vigorously licks them to stimulate their breathing and circulation. Her licking also helps remove membranes and messy birth fluids from the pups. Licking at this stage has a powerful bonding purpose, mother dog recognizes the puppies as her own and her licking comforts the pups. Mama's licking also acts as a natural waste disposal system, stimulating the pups to eliminate and keeping the whelping area clean and odor-free.
A Quest for Food
The behavior of licking faces is reminiscent of the bygone era when pups were raised in the wild and lived in dens. Back then, canine mothers obviously couldn't carry a dead carcass to the den. Instead, they would eat to their heart's content and upon returning to the den, the pups would lick mama's lips in hopes of eliciting her to regurgitate some food. This ancient form of "puppy mush" provided the puppies with nourishment until weaned.
A Welcoming Gesture
As puppies grow, licking behaviors learned from the litter may become a part of their behavior repertoire. Among dogs, facial licking is often used to welcome other dogs back into their social group and to signal appeasement. It's not unusual to see puppies lick the face of older dogs while holding the ears back and body lowered. Similar body language is seen when dogs are greeting owners.
A Cleaning Gesture
Sometimes dogs lick just because human faces taste yummy. While humans obviously won't regurgitate for them, dogs find the facial area appealing because of tasty remnants of food, intriguing smells or because human skin tastes salty. More than cleaning faces though, dog saliva can bring bacteria. Theoretically, humans can become infected by bacteria found in dog saliva, yet, there's little data to support that, explains veterinarian Ken Tudor. Parasites such as Giardia and Cryptosporidia though may pose a greater risk especially in children, the elderly or the immunocompromised.
A History of Reinforcement
Many dog behaviors are reinforced through positive feedback given under the form of attention. If every time your dog licks your face, you talk to him, pet him and provide any form of attention your dog perceives as positive, the licking behavior will be kept alive by such reinforcement. To extinguish this slobbery habit, stop fueling it with attention. Try ignoring it and replacing it with a more acceptable behavior that yields attention. For a more polite greeting, ask your dog to shake paws instead. With patience, time and consistency, you'll eventually enjoy more paw shaking and less licking.