Here's a story (and a video) of a homeless dog behaving completely altruistically. He (or she) dragged a dog that had been hit out of heavy traffic.
As mentioned earlier, dogs and humans have been companion species for about 135,000 year, it now appears. What benefits have accrued? Dogs can read human expressions. As proved by scientists. Humans can also read dog expressions. As proved by me.
Thanks to my daughter apricot blossom for showing me this video.