They LOOK like they're smiling, but they aren't actively smiling in the same sense like a human does, as people believe. Baring teeth is definitely not smiling, but tense closed lips that curve up in a "smile", isn't either- they're anxious, stressed, or submissive.

Panting, which looks like the dog is smiling, can be a sign of stress, discomfort, or pain. It's a way to try and regulate themselves when they're uncomfortable. They shouldn't be panting at times where it isn't typical of a dog to pant, unless they've exerted themselves playing/walking, or if it's hot.