Hate to spoil things for youse, but I think birthday dog may have a manipulated smile.
Dog body language is different from ours. To them, a smile showing teeth is a sign of aggressive intentions. Dogs show their teeth when they're about to fight. You shouldn't smile at a dog when he's angry.
It's still a funny picture to humans, though.
... or it could simply be the ban from the hat pulling up the cheek a lil.