My dog is a 10month old German Shepard/golden retriever/lab, he is male and not currently fixed. He has always been slightly aggressive with things such as toys and food. He often will go up to my parents dog and steal the toy that the other dog has. And if a dog comes up to him while he has a toy, he will growl. He does not do this with humans, and I can easily take his toys away or touch his food. He loves other dogs and loves to play, but as soon as something as small as a stick becomes part of it, he becomes aggressive. He recently got in a pretty aggressive fight with two different dogs, one new one(an 8 week old puppy) and one older dog who he is very familiar with. It seemed to be territorial/resource guarding. It is very hard to manage because it comes out of no where and very quickly, I try my best to keep all toys out of play time but sometimes it still happens. I just want to know what could be causing this behaviour and how to fix it, as he is the only dog in the household so I cannot practice regularly. Could it be because he is not fixed? I am aware resource guarding is a natural instinct but he is not only growling or even warning the other dog, it instantly becomes a fight and he lunges. Thank you and all suggestions are helpful.