Some might find this answer a bit ‘icky’, but it is fascinating all the same. A wild or feral dog, and some house dogs as well, when weaning their pups from milk to solid food, begin by regurgitating chewed up food for them. Hungry puppies will lick their mom’s face to stimulate them to regurgitate. That’s where the face licking instinct begins. Obviously, your dog is not licking your face for the same reason, but that early instinct gets tied up in a love bond and knowing that humans have taken on the mom’s role of loving and caring and providing for them. That lick is their way of expressing that they know that, and love you for it.