A dog will lick human faces as a way of displaying its status, feelings and to find out about the human whom the dog is licking.
Dogs lick human faces to indicate affection, be submissive and to sense information. In an affectionate situation, a dog that feels accepted may indicate both affection and submission as it licks the face of the humans with which it lives. In the wild, a dog or wolf will lick the face of a dominant dog as a way of begging for food.
A dog’s nose and mouth have evolved to be extremely sensitive to scents and the information contained in scents. Such scented molecules are carried by humans and are excreted by the sweat glands.
Dogs can process complex information about scents because of the millions of scent receptor cells contained in its nose and just at the beginning of roof of its mouth.
Human pheromones have been found in areas where there is body hair, the chest and face. Studies have been done on human pheromones which have been collected on the upper lip.
It could be that the dog is sniffing and processing the information contained in the oily material that released by the apocrine glands that are located in the ears. Eccrine glands or sweat glands are located on the human forehead. Together, the apocrine and the eccrine glands can release complex information about the human that the dog is licking.
When your dog licks your face, it is able to gather the information and draw it into its mouth where there is a vomeronasal organ which processes scents. Your dog can receive messages about you that you send involuntarily. Your dog, as a member of your family, feels that it has the right to gather such information about you.