German Shepherds Need an Alpha:

True German Shepherds. (They were litterally bred for 'suspicion and aggression')
Without proper training - 100% of German Shepherds end up as something the owner has to "explain" - - to wit: "He's not good with strangers, avert your eyes, hold out your hand and wait. And if he likes you then you can keep the hand." Like some kind of Harry Potter Hippogryph meeting. Jeez.

Or you can socialize them and more importantly: be an Alpha to them, and make them truly the best dog you've ever had besides the shedding.

Here's the thing: Take a plain dog and breed them for walking the grounds in a mighty fortress.
"We need a dog that's smart as a whip". Check [x]
"We need that dog to be vigilant and suspicious". Check [x]
"We need that dog to fight, not run". Check [x]
"We need that dog to obey it's master." Check [x]
And you get the German Shepherd

But if you never get established as master ----- oh lord, you've got a guard dog that's smart, supicious and is genetically coded to fight and not run. It ain't the dog's fault.

Then for the non-Alpha owner: It's just all about protecting everybody the dog doesn't trust, which is anybody but you. Because if you're not their parent and Alpha, then you're their Ward.

"He's not good with strangers, avert your eyes, hold out your hand and wait. And if he likes you then you can keep the hand." Like some kind of Harry Potter Hippogryph meeting.

Okay if you got me a German Shepherd, I'd be delighted. It'd be skinny its whole life because they have terrible 'back ends' that wear out WAY too soon. They'd chew on beef bones a couple times a week to stay off the dental-table. I'd reach out to a trainer who still believes in Alpha training. NOT Alpha domination. But still, Alpha.
I'd get a good book like "The Monks of New Skete" "How To Raise and Train Your Own Best Friend" or something like that. And I'd avoid dog parks like the plague until the shots were 'done'.
Sandy at DogSchool 101 loves German Shepherds but I don't know if she still trains Alpha
A lot of trainers have changed to 100% positive reinforcement which is this:
Your dog is killing a cat. You can't say "NO!!! BAD BOY".
You have to wait until it STOPS tearing up the cat, and say "THAT'S a GOOD BOY" See how he's praised for doing good things? NO emotional damage.
They're DOGS. They're smarter and more "black and white" than we are and that's why they've been around for 1.2 million years longer than us.