" Even the submissive grin is misunderstood. Sadly, it can be mistaken for a snarl, and a dog may be labeled as aggressive who is actually anything but. It’s also often perceived as a doggy version of a happy smile—a less damaging interpretation, but still a misperception of a clearly subordinate display. Interestingly, the submissive grin is believed to be an imitation of the human smile, since dogs don’t normally display this behavior to each other, only to humans. While some behaviorists consider the grin to be an attention-seeking appeasement gesture, others consider it more of a threat-averting deference signal. In any case, it’s important to understand that the dog who grins is making a status statement—your rank is higher than hers—exhibiting neither an aggressive threat nor a relaxed, contented smile. "

― , The Power of Positive Dog Training