It was once believed that the human laughter was what set us apart from other mammals. We actually learned to laugh before we ever developed methods of speech, between 2-4 million years ago. However mammals in general are social creatures, more so than birds, fish, etc and so they PLAY just like we do. Following this logic scientists have known for some time, like DARWINIAN time, that primates do laugh, and postulate that other mammals did too, just not the way we thought, and not always for the same reasons humans do. In fact in the 90s it was proven that rats will “laugh” if you tickle them, but strangely not mice – they are more solitary creatures though. See the article/video at http://www.wired.com/wiredscience/2013/09/tickling-rats-for-science/
, Laughter for humans also tends to be a “social” construct, and having a sense of humor does seem to differentiate us from other mammals, even other humans. We are 30x more likely to laugh in a group situation than alone, over the same subject matter. Which is why comedies have laugh tracs, to stimulate your funny bone. Talkers laugh more than listeners. Women laugh more than men. Psychologically laughter is a way we as humans signal our empathy and understanding and share intimacy and like-mindedness with the “many”. Our voices will even achieve harmonic resonance when laughing in a crowd. And human laughter is complex, we have many distinctive kinds of laughs with different meanings from amusement to embarrassment to contempt. See this article for some of those http://www.scientificamerican.com/article.cfm?id=rats-laugh-but-not-like-human
It is also can provide an emotional bridge, so laughter in a painful or distressing situation gives the brain a moment to decompress and refocus. Of course the chemical benefits of laughter can be found all over the internet so I won’t go into that here, but suffice it to say that a sign a relationship is floundering, is when you are no longer laughing together.
Laughter can not only bridge human gaps though, it also connects us to other species. Remember those rats, well they more closely bonded to the humans that tickled them, trying to instigate play more often from them etc. So all those times you just knew your dog was laughing at you, or with you, it probably was, and you can learn to join in… See the articles, and go out and have yourself a good hearty laugh!!!