A Dog's Mouth Is Cleaner Than a Human's
"The belief that a dog's mouth carries fewer or less harmful germs than a human's, often used to justify letting a dog lick a wound or your face."
Dogs' mouths carry roughly as many bacterial species as human mouths do, somewhere around 600 in dogs compared with over 600 documented in humans, so "cleaner" in the sense of having fewer germs simply isn't accurate. The real difference is that most of a dog's oral bacteria are specifically adapted to live in dogs and don't easily colonise humans, which is probably where the "cleaner" reputation comes from: cross-species infection from casual contact is genuinely uncommon. That's different from harmless, though. A dog bite that breaks the skin carries a real risk of infection from bacteria dogs do host, and the old observation that dogs heal well after licking their own wounds isn't unique to dogs either, saliva from most mammals, humans included, shows some of the same mild wound-healing properties.