Some dogs are born easily over-aroused, and can be a “bit much.”
(Some dogs are really unlucky, and are both born easily over-aroused, and experience common dog-related traumas ( are breeding dogs, are placed improperly and are re-homed multiple times, are sent to “boot camp” where they are shocked and kept in cold kennels, etc.)
Often, these dogs like certain dogs and bond closely with them, and on the flip side can be very wary of other dogs, even reactive.
When a dog has thoughtful guardians who follow this sort of muzzle-training program AND are careful about what sort of situations they put their dog in, often over time with their dog muzzled for example on woods walks or time at home with a “roomate” dog or a visiting dog.
When a dog is spared from really stressful situations, is muzzle trained, and is granted more freedom of movement than before, their dog will likely have sporadic interactions with a few other dogs. In these interactions, their dog will realize that biting is not an option. In low-stakes situations, the dog will say to themselves (I am guessing here), “Hmmm, usually I would come in HOT and bite, but now I can’t… what are my alternatives?” Over time, such dogs can develop really wonderful social coping mechanisms. They can even re-socialize!
Don’t be discouraged by relapses into aggressive behavior. Work with a behavior professional if you are really struggling, and know that if your dog loves their muzzle, the future can be really bright. A behavior professional can help you understand what situations are, and aren’t appropriate to put your newly muzzled trained dog into.