Our GSD, Gurney, was very aggressive with my border collie, Lass, ripping her face open twice in a week. She was the most subservient dog I have ever met and wouldn't say boo to a sparrow (not a goose - I wouldn't say boo to a goose - which I think encouraged him. We had him neutered and he was a different dog, very gentle and loving. In fact, when he was quite young and at nose level with my grandson, then aged 3, the lad punched him on the nose. I really thought the dog would go for him but he just shook his head and walked away.
The only time he showed any sign of aggressiveness was when we were away over night once. When this happened, our neighbour would go in several times a day, using the side door, and walk the dogs and feed them. Another friend would go to see to the goat, using the front door. this particular time I Had given her a side door key and Gurney growled and showed his teeth at her. I am sure it was because she came in the "wrong door".