Originally Posted by
The Wife
Mine started a bit of growling at around 6 months. He was definitely trying to exert his dominance and push boundaries. During this time, he would also test us by not following simple commands, like "come" or "sit". (That could possibly be a way of figuring out if he's testing his boundaries or if there's another problem.) It took a bit of patience at times. When he started growling, it was game over -- we would stop what we were doing and say "hey" roughly, looking him right in the eyes until he backed off. Once he showed submission, he would get attention again.
It's startling when it happens -- this is your puppy, right -- but it's important to deal with it right away. You have to remain the dominant one. That said, we've been taught by trainers that if a dog gets aggressive, you grab it my the nose and bring it to the floor (without hurting, of course) until it whimpers in submission. We were taught that this would be what its mother would do. I'm sure there are several opinions out there on this method...