The Boston Terrier comes from tough and ferocious dogs. The Bulldogs were developed to work with butchers to engage and control steers and cattle several times their size and could cause fatal injuries if the dog was not fast enough or hard enough. The burrows that contributed their genes to the Boston Family Reserve were melee fighting dogs, fearless fighters, but totally harmless to humans. It is difficult to explain or understand how these ferocious breeds were used to create the friendly and happy companion we know today. But over the generations, dogs have been selected as much for their relaxed, non-aggressive nature as for their type and structure.
Boston Terriers can be aggressive towards other dogs, especially when they are behind a fence or leash. It can be something the dog has learned, or it can be a genetic system. Whatever its origins, this behavior is not typical of Boston Terriers and should not be considered acceptable. In many cases, this behavior can be changed by teaching the dog to focus on his owner and rewarding him with praise and acts for ignoring the other or the other dogs. Under no circumstances should a dog be punished or corrected for this aggressive behavior by pulling on the leash, shouting after the dog or hitting it. Aggressive behavior is usually caused by fear, and punishment can increase the fear and hence the aggression to which the punishment was supposed to end.
If your Boston Terrier displays aggressive behavior towards you, humans or other dogs, you should have a competent dog trainer or behavioral specialist who will work with you to change the dog's behavior without resorting to a sanction or correction of any kind. Before the start of training, the coach or specialist will probably recommend a thorough veterinary exam to eliminate the physical causes of aggressive behavior. Sometimes the pain or illness can cause a dog that has always been peaceful to suffer a sudden aggression. Some chemical imbalances in the body, such as low levels of thyroid hormones or liver abnormalities, can trigger aggressive behavior in dogs. I saw a few cases of Boston terriers that "went" suddenly and unconsciously "bite" on their owners and bit them severely, and every case of this type was attributed to a brain disorder that caused seizures.
Aggressive behavior is not normal in Boston Terriers. A Boston who develops an aggression must be thoroughly controlled by a veterinarian and put on a positive behavior modification program, based on rewards, under the supervision of an experienced dog trainer, a behavioral specialist or a licensed veterinary behavior specialist.
Ask your dog-dog friends for recommendations on dog trainers. Call the trainers to ask when you forget their classes (if they say no, look elsewhere). Look for a coach who is very positive, who focuses on excellence and rewards good dog behavior and avoids punishment or correction for bad behavior.