Arguably the most common cause of the issues dog owners contact me for help with stems from them having the wrong breed of dog and the two of them being a mismatch; a mismatch between what the dog they have is and what it is hard-wired for, and the situation the dog finds itself in with the owner.
A breed can be wrong for someone for many reasons but the most common I see are it being wrong for the owner's handling ability and experience; wrong for the amount of time and resources the owner can provide to the dog; wrong for the owner's living situation; wrong for the owner's idea of what this given breed is or should be; and wrong for what would actually suit the owner, rather than suit what the owner wants. As you can see these issues are all wrong on the part of the owner, not the dog.
If a high-drive dog finds itself being kept in a home with no opportunity to express and satisfy itself through behaviours appropriate to its breed and it becomes destructive or begins to display undesirable behaviours, the fault lies with the owner who keeps the dog there under such conditions, not with the dog. Working breeds in pet homes are prime examples of this incompatibility. Working dogs are supposed to work, not be kept simply as pets, so it's no wonder why your German Shepherd*, Belgian Malinois, Dutch Herder, Rottweiler, Giant Schnauzer, Dobermann, livestock guardian or even your working-line Cocker Spaniel hasn't quite met your expectations as a pet. If your plan is to live the typical London-pet-owner lifestyle with your dog, taking casual walks in the park, freely allowing your dog to interact with strange people and dogs and enjoying the dog-friendly cafes and pubs that London has to offer, then chances are a breed which has been bred to have a defensive edge or to chase and bite isn't right for you.
Choosing a breed that is appropriate to you, your ability, your living situation and the life you can and intend to provide that dog is paramount. This of course begins by knowing the breed of dog before you bring it into your home; the function they were intended and bred for; the drives that gear them toward those functions; by having first-hand experience of what is required to properly care for, manage and satisfy them, and by making sure you can and will provide this for the dog's lifetime. If you do not intend to use a breed for what it was intended for, or something similar i.e your working-line German Shepherd doesn't have to be a police dog but you may train for a breed-appropriate sport such as Shutzhund/IGP, then do not get that breed. Choose a breed better suited to your intentions for the dog and more appropriate to your situation.
Something worth mentioning as a key aspect of being able to handle a working breed, particularly the larger ones, is an owner's physical ability to handle them. Yes, in time the dog can be trained to be obedient on commands alone but more likely than not it will have grown to almost full size while still retaining its puppy levels of energy and excitement before your training reaches this level, and it only takes a couple of handling failures to allow for significant reinforcement of undesirable behaviours to happen to set your dog off down the wrong path. Think ahead to the dog being an adolescent and ask yourself if you can really handle a powerful 40kg German Shepherd that decides it doesn't want to listen to you.
Responsible dog ownership starts with truly knowing what breeds are suited to you and what you can offer a dog and then choosing one of them, rather than choosing a breed because you like the idea of them or because of how they look. Stronger breeds may in time become better suited to you as your handling ability and experience develops and you have the capacity to really do right by a dog of that breed, but you shouldn't jump in the deep end and try and figure things out as you go as there is a good chance this could come back to bite you or someone else in the arse.
What was the breed bred for? Does that align with the life you can provide a dog? If not, then do not get that breed.
*I should mention that all of this applies to choosing the right breeding lines within a breed too, as a healthy show-line or lower-drive working-line German Shepherd may suit your family's active lifestyle in Greater London, but a German Shepherd from police or security lines will likely be volatile towards everyone bar your family, so no nice walks in the park and say goodbye to being able to relax when guests visit your home. A good breeder should be able to advise and properly allocate a suitable puppy to you, but there's huge emphasis on the word 'good' here.
This is of Shwan, a Caucasian Shepherd who at 7 months old was already in excess of 50kg! Bred to protect livestock from wolves and bears in the Caucasus mountains, Shwan was a complete mismatch to the situaiton he found himself in, living in a terraced house in West London. However, Shwan is my greatest rehoming success story as with the support of his very understanding and responsible previous owners and the help of Graham from Caucasian Shepherd Rescue, Shwan now roams freely around 80 acres of fenced woodland in the hills of Abderdeen.