peacemakers?

I have 3 dogs who are all friendly peaceful animals. Lately I have noticed a pattern> The oldest dog has become a little aggressive towards strange dogs. Today we were all out walking and the neighbors big dog and my dog were growling and circling each other. My young dog went and got between them much like much like a person trying to get two angry people to calm down. It worked. I am curious as to why he broke up the budding conflict and how his actions worked. It seems very similar to primate actions. Is he making peace so h wont have to fight? is he being altruistic? I look forward to seeing more such interactions