Keanu, my now 3 year old GSD, has been doing well socially, and i'm proud of him for it. However, just recently, he's been doing something....and i'm not sure if its protective or aggression that needs working on. I'm hoping its a protective thing. Anyhow, just today, as an example, my 27 year old brother was rough housing with my 15 year old brother, in the garage/second den, and Keanu was in there. He might have been trying to stop them, but he jumped up behind my 15 year old brother and nipped him in the back. I'm just not sure what to make if it or how to get him to stop doing it, just short of asking them not to rough house with him in the room. Any ideas?