I am pretty sure he is territorial. He is like that just at home. Barks at other dogs when hes in the car as well. Barks at everything when looking out the window at home. No difference between the sizes of groups. However, when we put a muzzle on him he is very sweet and kind (most often). When I was in college he stayed away from people and was afraid. Gradually he became more agressive and started to go after people. I will give you an example of his behavior. Myself and two friends were sitting in my living room. T.O. at my side laying peacefully. One friend went up to get a beer and he went after him. Luckily, I was able to snatch him in midair. We have taken him to classes and programs that feature other animals as well as humans. He usually does pretty well controlling himself. He is now living in his 4th home as well if that makes any difference. (2 college apartments, parents house, my house currently)