Socialization missed during puppyhood is the cause of the vast majority of reported adult dog behavior problems. As an example, the most frequently reported GSD behavior problem on our site: Say, your dog becomes aggressive and incessantly barks when other dogs or people come close. Then this has nothing to do with heredity, and typically it has nothing to do with a trauma. Instead, this commonly reported dog behavior problem is purely down to missed socialization during upbringing, during puppyhood.
The reason why GSD owners report this behavior more often as a problem is this dog's inbred psychology: to react to environmental stimuli. This is why a German Shepherd puppy needs to be socialized much more comprehensively than other puppies, or else as an adult dog (s)he will feel alerted and thus stressed upon so much as a distant neighbor mowing his lawn.
So if the vast majority of adult dog behavior problems reported by GSD owners are down to missed socialization during puppyhood, the question is: Can we make up for what we or the prior owner missed? Can we still socialize the adult dog?
The answer is: Yes we can, and when comprehensive puppy socialization was missed we must focus on dog socialization. Just be aware that socializing an adult dog takes much more effort than socializing a puppy! Same as with people: Think how many friends you made as a child and at school and even during college or higher education, and how few friends you make as adult - which by the way is the very reason for the mere existence of "social media" sites, and why many of them use the term "friend" or at least "follower"; it's a psychological substitute.
See it this way: When your dog never learned from experience that all dogs are good dogs and all people are good people unless they behave badly (say, threaten you or the dog, or whatever), then naturally your dog will see every approaching dog and every approaching person as a potential threat! That is no fun for you, and certainly no fun for your dog. The dog will be stressed out 24/7! That will make your dog very sick and the dog's behavior very unpleasant.
When your dog learned from experience to see every approaching dog and every approaching person as a potential threat, then obviously it will require significant effort to reverse this "trained" behavior for an adult dog. You see here that "trained" behavior does not require our consciousness at all. In fact, most behaviors that an adult dog assumes neither we nor a prior owner have consciously trained! We may not be aware, but however we behave, the dog learns from that. The dog adapts.
Now you know that your adult dog's behavior problem likely is down to missed socialization, and why comprehensive socialization is crucial for the German Shepherd Dog. And frankly, even in case your adult dog's behavior problem is not down to missed socialization, we cannot socialize too much. So go and start socializing your dog more comprehensively now.
If you are wondering "How? How can I comprehensively socialize my dog?", note that the Puppy Development Guide (obviously) discusses socialization in detail (and even includes a socialization checklist). Remember that for your adult dog the socialization will require more effort though, so show some patience.
If you prefer to see a lot of socialization being done live, be aware that the Live and Multimedia New Puppy Diary shows and explains everything.
Note that every key point raised above you can find more comprehensively explained in other places on this website. The menu is your friend. Here, links have been omitted only to keep this decision tree straightforward.
PLEASE NOTE: Posting a fragment of your overall dog problem in the comments below is not going to help. Provide complete details if you really seek the right solution. Of course we have a page for that as well: Dog Problem Consultation.