Pug sneer
The first pug is doing a good job “grinning to the camera,” while the second isn’t. The former appears to be smiling. To indicate and others that he’s not a danger, the latter is performing an appeasement gesture called a submissive grin, which shows you and others that he isn’t dangerous. Submissive grins are a form of tribute used by dogs to show respect for another person.
The submissive grin is a sign of submission used to deflect human aggression. The behavior is often seen in greeting situations where the dog is demonstrating interest in a non-threatening manner. It must be rather comforting to have one of these!