I am a long-time German Shepherd Dog (GSD) owner and can tell you that they are one of the most intelligent, yet territorial dog breeds in the world. As such, they have been placed on the "naughty list" by most insurance companies. This is something to keep in mind if you are considering adopting one. Back to your question, I believe that GSDs are born with a territorial instinct; however, their level of assertiveness and aggressiveness really depends on the environment in which they were raised. Eight months seems to be the age when GSDs become more aware of their surroundings and start to develop a "that's mine" attitude. It's best to introduce the breed to strangers early on, so that they do not develop a fear or become too aggressive. Set boundaries and make sure to stick to them. GSDs know how to problem solve. They also know how to get the upper hand. Good luck!