If you spend time working with the dog you have, you will grow to appreciate them for who they are, and be able to maintain and design and incredible working relationship:)
I have said before, I do not see dogs as a listing of their faults as their total personality or "the real dog". I always see dogs as them plus their potential. That being said, not every dog's potential is to be the idealized "bomb proof" dog who loves everything and anybody no matter what. A dog has a very statistically high probability of dramatic improvement in some way. If we get beyond the point expected for the team, I will gladly take that. Sometimes and not often a dog does much better than I imagined for them. Owners should take that kind of success, if it happens but should know not to expect more of their dog than they can deliver realistically.
If you have stopped working with a behaviorally challenged dog, realistically you should expect the behavior will revert back. Some people call this "unpredictable", but trainers who work with dogs know that is highly predictable and likely. An owner can not get to a level, and then drop back to no social interaction, no training ET, and expect the dog to be fine later on when put into a situation that is no longer familiar to them.
Too many owners feel that because a dog now gets along well with the dog that lives in their home, suddenly they are now opened up to meeting all sorts of strange dogs. If you have a dog that does not like dogs getting into their face, and has not relaxed later on (though is in control around these dogs) with strange dogs getting in their face----please do not think your dog is now fine to have loose, not under command, and suddenly like strange dogs doing this. It is not unpredictable that something bad might happen, it is a highly predictable event. The owner is still responsible to manage the environment and work the training to keep their dog and others safe (or humans safe). This does not matter if the dog coming up is cute and friendly, if the behaviorally challenged dog does not perceive the circumstances in this way.
Through socialization and practice the trainer or owner can find out that their dog does in fact like play, does like strange dogs. Sometimes their frustration at not being able to play comes across as aggressive actions as the dog gets too aroused, stimulated, and frustrated. Remember though, the DOG is coming to that conclusion under their own free will, a dog can not be forced to like playing with another dog. A skilled handler or trainer can bring out that natural inclination if it indeed exists.
Some dogs are not going to like other dogs, especially strange dogs, coming right into their face. Often, one can train these dogs to be controllable and learn not to react when given a directive from their owners. This requires a lot of serious and consistent training. A dedicated owner is necessary for this to work. It takes a lot more work than if you happen to own a dog that really does like other dogs. Stress is involved in owning and working a dog like this. Again, dogs are more than their behavioral issues to those who love them. The rest of the dog is worth the additional work needed to get them to a safer and consistent base. When they are there and in control, this does not mean an owner can sit back, put up their feet, and forget what their dog is up to or doing, especially in new and different situations.
I have owned two dogs like this, Jackie CD and Neptune. So I am really aware of what it takes to do this. Sometimes owners do not listen or heed my warnings and an event occurs. It usually occurs when they decide it's okay to let Ginger meet the friendly dog after all. Sometimes it is because the owner is embarrassed that they do not own the kind of dog who they could expect to greet "miscellaneous friendly dog" politely, and they give into the peer pressure of the other owner who assures that their dog is friendly. The point is, it does not matter that "miscellaneous friendly dog" is friendly or not, what matters is how your dog perceives the situation. If you release them into "miscellaneous friendly dog's" space to their own devices, you have just given up control of your dog and are now hoping that your dog is not going to repeat the predictable behavior that they have historically done in the past. THAT does not work for the dog. They are not reading your mind, and we are not able to tell them in English that we would prefer if they did not bite "miscellaneous friendly dog".
Trainers always try to explain to clients the clarity of being able to tell your dog WHAT to do rather than trying to have them guess what we want them NOT TO DO. Also, when walking a potentially aggressive dog in control around other people, we can not expect those humans to know what our dog is capable of. That is the owner's responsibility to know their dog, and to explain that to a public that will erroneously release their friendly dog towards you. The general public expects all potentially aggressive dogs to look wildly out of control, and be openly aggressive. The only one that can let someone in on the hidden behavioral challenges of their dog is their owner.
Some owners will be so proud of what they have accomplished, they drop the training, and remain in complete denial about some aspects of their dogs personality. They believed that even though their dog never showed an acceptance of strange dogs, suddenly their dog is going to be able to handle this (out of the blue so to speak). So suddenly their dog is released in social situations that they would never have been while training was going on. That is not a good combination to have. It virtually guarantees that an incident will happen in the future. To the dog owner this guaranteed incident feels "unpredictable" when it happens. They feel they can no longer trust their dog.
That is a shame, because what they can trust and predict is that their dog never did and still does not like dogs getting in their face. Should you not prepare them or release them to do "as they will", it really should be no surprise that they will exhibit the aggressive behavior.
It really happens with all sorts of behavioral issues outside of aggression, but aggression is what will have dog owners giving up on their dogs, so that is what I am concentrating on in this post. If you go for years with a potentially aggressive dog that has not had incidents, this is usually due to training continuing, management of the environment (IE "no my dog is not friendly, my dog can not meet yours and your dog can not come over", muzzles and such). Just because the incidents did not occur does not mean that your dog suddenly loves every strange dog on the planet. The ones who do decide they are into strange dogs are pretty obvious about it. Though one point I should make is that their behavior can vary depending on the handler in whose hands they feel the most safe. I have at least one client's dogs who comes here, and their owner can not make them feel safe enough to release them to play, while the dog views me and my place as a safe zone. This does not frequently happen, but it can happen that a dog does not feel safe with their owner. Maybe because their owner is not that good at reading other dogs and/or their own dog. Lots of reasons may come into play for this, but again it is usually fairly obvious when this happens. So if this client suddenly decides that just because their dog is relaxed enough to do this here, that their dog can suddenly do this with them without the additional work and training (in this case to train the human), they are fooling themselves.