What does the science say? In 2012, a group of neuroscientists examined findings about neurological substrates in human and non-human animals and finally declared that animals have consciousness and emotions. So, having emotions as a non-human animal is definitely possible. And when a dog feels content, he has relaxed body language - this means that his facial muscles are relaxed, making his mouth open and the corners of his mouth turn upwards, according to the ASPCA.
But K.C. Theisen, director of pet care issues for the Humane Society of the United States, urges us not to be too sure of every smile. "A dog who has the open mouth, the tongue hanging out, and the wrinkly cheeks is not necessarily a 'smile' in the way we think of a smile," Theisen told The Dodo, cautioning that misinterpreting the expression could end up in a bad canine encounter. "But almost all mammals have some sort of facial expression that they use when they are trying to be friendly. Learning what dog's body language can tell you is super important, since a dog 'smiling' may or may not be approachable and friendly." She added that if you want to approach a dog, it's always a good idea to ask the human chaperone first.