Another fun fact, dogs actually sigh when they are feeling very comfy and safe, it's their version of a purr. So a sigh isn't a stress response, it's a love and trust response.
Fun fact, did you know that dogs are the only animals that actually don't have to do anything for a living? All they got to do is just look at you with the cute eyes and they get fed for the day or other animals like cows. Have to do something in order for them to get fed for the day