I took Impulse to the vet today to get weighed. He walked in, put two paws up on the counter, and wrinkled up his nose, showing every one of his teeth to the woman seated there. Fortunately she remembered him, and knows that this is his way of saying "Hi, I'm thrilled to see you." People who don't know him sometimes take it to mean, "Hi, I'm going to remove the nearest portion of your anatomy now."
Smiling isn't a natural doggy expression; it's something some of them pick up from observing humans. And once you're sure that it IS a smile and not a snarl, Impy's smile is extremely cute. The problem for other people is that they often aren't quite sure which it is!