A grin for animals is not a smile. Usually, they show aggression to other creatures in this way. But pets, living with people and adopting their habits, including facial expressions, sometimes copy their owners.
The owner adopted a dog named Titlist from a shelter. When they found her, she was hungry and exhausted. It took the Titlist a long time to recover and start trusting people. But now she lives in France and she is doing well.
She does not stop smiling and making the owner laugh with her muzzle.