Well my dog is a pretty tough dog for being a small dog little fellow, he doesn't really care who or what he breaks at. The problem is he has a few years left to live, and being he doesn't like other dogs, he's been humping my leg like crazy gotta tell you guys.
Always hated that breed...
OT: Maybe it will like horses?
This is your dog letting other dogs know that hes protective over you and he will fight for your safety. Or else your dog feels as if he owns you.. Ether way its a protective state and territorial issues.
When he does stuff like this make him feel ashamed by saying stuff like " Stop! Don't do that! What are you doing!? " as crazy as it seems it works with my dog. He see's another dog and want's to run up to them and start sniffing them and such and i just make him feel ashamed and pull him back and he looks the other way.
LOL i think he just insecures to the other dogs. try to expose him to other dogs.