Yeah. It’s the showing teeth (or that thereof) that in many animals is a threat display. Think of it like a non-verbal way of saying “See these teeth? How’d you like them in your face? Because that’s gonna happen if you don’t step the fuck off.”

I find that my cats and dog DO smile with their mouths/faces closed when getting a particularly nice petting. It seems a bit subtle because they’re smaller than us but, rather like a little smirk (best is when my little cat gets a particularly nice nose scritching and he can’t keep his mouth closed because it feels too good and a loud purr escapes from​ between his little fangs).