Chewy.com indicates that dogs, especially young puppies, lick human faces as a warm greeting and a show of submission. Younger pups lick much more often than older dogs because licking is an instinctive maneuver puppies use with their mothers to request food.
The more calm and submissive a dog is to its owner, the more likely it is to lick a person's face often. Other factors that cause a dog to lick a person's face include a liking for the taste of the person's skin, or a lotion or food smell on the person's face. A dog may also lick sweat or tears from a person's face to enjoy the salty taste.