Dogs don’t really smile; They do show happiness, but not necessarily using their mouths.
Although dogs don’t actually smile like humans, they sometimes happen to make expressions that look like smiles; When a dog feels genuinely at ease, he may actually position his mouth in a way that truly resembles a smile.
Happy dogs have a general looseness to their bodies, and that applies to the mouth area, too. If the sides of your pooch’s mouth point slightly higher than the rest of it, that often – but not always – signifies that all is wonderful in your dog’s world for the moment. A loose-hanging tongue combined with a mouth slightly ajar generally points to a good moods in a doggy.
If your dogs’ mouth is open just a tad, with the sides raised, he may indeed look like he’s smiling, but he may actually be anxious, nervous or in distress. Signs of distress accompanying a stiff smile include heavy panting with the tongue in, whining and chattering teeth.
If a dog is threatened by another animal or human, he may attempt to show its subordination by raising his lips in a nonaggressive display. It’s a different baring of the teeth than an aggressive one. It has the appearance of a smile, but the dog is scared.