Some people say, it increases oxygen intake (because the inhalation is 1.5x - 2x a nomal breath when you sigh). Others, that dogs sigh for the same reason we do - when they're nice and comfy, when they're bored, or when they can't get you to do what they want!
Babies and infants sigh too, indicating that it's innate, rather than just a learned response. Scientific studies show that sighing in people can trigger a sense of relief, and "reset" the respiratory system to loosen up, as in this article below:
So in times of stress, when breathing is less variable, a sigh can reset the respiratory system and loosen the lung's air sacs, or alveoli, which may be accompanied by a sensation of relief, Vlemincx said.
Knowing this, it would seem logical then to add some sighs to the breathing regimes of people on mechanical, ventilators. As it turns out, it has been tried.
"If you put in a few sigh breaths, people feel better," said Frank Wilhelm a clinical psychologist at the Universität Basel in Switzerland.
As with people excessive sighing can be a cause of something deeper (in people, hyperventilation issues) - so have this checked out if you're disturbed by how often your dog is sighing!
In our first few days with Lia, we were tickled pink by her sighs - loud, loooong and heavy - she sounded like she had the weight of the world on her, and really, can't things just work better around here! This is a very distinct sound from her grumble - a grumbly sound she makes when we've left her for too long, or when she knows we're at the door outside (and she's inside wagging her tail) but spend more than a minute talking to the neighbours and not coming straight in for a hello ("honestly! where are your manners! come in already!" is what she's trying to channel I think).
Lia sighs when she's about to go to sleep (this is a nice sort of relaxed sigh, usually coupled with her moving her jaws like she's masticating her dentures), and also when we make her sit-stay for her meals a little too long for her patience. She sighs generally when she's given up all hope for us doing the right thing. She's such a grump! Just like us, really.