Well, from what I understand, there's the scientific/evolutionary reason, and then there's the "romantic" idea. Basically, the theory goes, I think, that humans domesticated dogs thousands of years ago and each species learned to depend on the other for certain things, i.e., dogs would protect/warn the humans from danger and then get food. This is really simplistic, admittedly and I'm not an expert but have done some reading on it. I think that over the centuries many dogs have really evolved almost to the point of being persons in that they clearly show emotions, affection, caring, love. I know our dog did. I was lucky she was still alive when I got PTSD. So many days I'd be lying on the couch crying and she would sit next to me, put her paw on me, lick me, try to distract me and care for me or just simply be there for me (she is my avatar, by the way). I am sure, though, that not all dogs are therapeutic for people. Each one is an individual, just like we are, with their own histories, personalities, experiences, backgrounds, psychological makeup, etc. I hope someone with more knowledge addresses this.
