In case anybody is wondering the actual reason, it's because many quadrupedal mammals bathe by licking themselves and each other, particular between parents and offspring, so a dog licking you is just the dog's instinctual response to close physical contact with family members. Some hypothesize that dogs enjoy being pet because it emulates the sensation of their mother licking them. Some animals are somewhat incapable of cleaning themselves at a young age and that leads to problems with early separation, such as Lions who in captivity are often paired with a dog who serves as a surrogate mother for cleaning them.