An obvious discomfort on the first walk together...

Here are the most common ones:

• She's afraid of another dog; German Shepherd who is simply afraid of another dog tries to cover up this fact and displays aggressive behaviour; her tail would be probably low or even between her legs and hackles high up. She would be close to you or even behind you when another dog comes closer. In this case you need to build her confidence;
• She simply doesn't know how to interact with another dog;  German Shepherd who had very limited exposure and interaction with other dogs very often believes that this is the proper way to interact with a strange dog. German Shepherd Dogs are very family/pack oriented and their instincts tell them that strangers both human and animals are something to watch for and be cautious around and she does not know the proper ways to express her concerns. She needs time and patience to understand that you are in control and all she needs to do is rely on your leadership;
• She doesn't like the attitude another dog is displaying;  We often see an arrogant attitude in a strange dog (often happens in a leash free areas). This dog insists on initiating the "sniffing" and the play, regardless of signals our dog is sending. Our dog has a full right to tell the other dog how she feels about him;

• She had a bad experience with another dog as a youngster and now hates a particular breed(s) or other dogs in general;  It is very important not to expose puppies to any situation that might leave a negative imprint. All initial experiences should be wonderful and positive, first introductions should be to other puppies of similar size and age... It is our responsibility not to place our youngster in the situation that would be overwhelming for her. It is much safer not to give the puppy an opportunity to play with strange dogs at all than risk having your pup 'corrected' by a stranger. There is absolutely no need for the pup to play with other dogs, as long as you provide her with plenty of opportunities to see and meet other dogs in a safe, on leash context. Here again she needs time and patience to understand that you are in control and all she needs to do is rely on your leadership and your ability to deal with any situation;
• She loves confrontations and gets her high from interacting aggressively with another dog;  This is not a very common problem within a pet population, but does happen when an irresponsible breeder sells the dog with a high fight drive to an unexperienced person;
• She is territorial and another dog enters what she perceives as her territory;  This is a common problem when a dog without clearly defined responsibilities believes that it is her job to keep ALL strangers away from her owner and home;
• She feels your discomfort when another dog approaches and tries to 'fix the situation';  This is probably the most common problem in the pet population. When a young, still insecure German Shepherd Dog starts barking at another dog, which is an absolutely normal phase in her development, an owner panics, grabs the dog, pulls her close, maybe corrects her. Next time when a strange dogs appears on the horizon, the owner becomes nervous, grabs the dog, tightens the leash and starts saying no, no... even before the dog himself had a chance to see the approaching dog. The moment the owner becomes concerned, his German Shepherd becomes instantly concerned as well. She associates an approaching dog as a source of her owner's concerns, so of course she tries to assure the owner that she is able to take care of the situation, that she is in full control and there is no way she would allow another dog to do anything bad... which in turn makes an owner even more nervous and the problem escalates... And it all started with miscommunication;
• She is bored and barking and lunging at other dogs is a form of entertainment;  This is self explanatory;
  
• She is frustrated about other issues and confrontation with another dog is a form of release for her;  This is probably most frequent case when the German Shepherd Dog is over-trained or over-corrected and when the dog's simple needs of intensive experiences are not met. Look at the intensity in your dog when she's lounging and straining at another dog. Do you give her an opportunity to experience this level of intensity during play?
  

Any variations and combinations of the above;

And probably some other reasons I can't think of at this moment;

In majority of cases you have to work on the confidence and comfort around other dogs. You start in a comfortable distance. The moment YOU see the dog on a horizon or turning the corner you start feeding your dog the treats or bring out the favorite toy. Step out of the other dog’s way by backing into the driveway, or crossing the street continuously dispensing the treats or playing with the toy... the idea is to keep the dog really BUSY with the treats or the toy. Position yourself at first in such a way that your dog’s back is turned toward approaching dog. It is amazing to see how soon the dog’s reaction to approaching another dog is to turn instantly toward you!!!