Socialize
Aggressive dogs, (assuming they were not trained to be aggressive) are almost always the result of a lack of socialization. If you have a new puppy I implore you to take them to the dog park, everyday. At least for the first few months. Watch them closely and observe how they interact with other breeds, sizes, ages and temperaments. It’s imperative that dogs learn to play as a puppy. And yes: learn. A puppy needs to learn how to play; it is not inherent. Sure, some dogs may naturally be docile or passive, but especially those dogs must learn how to play.
And if this opportunity is missed as a puppy -it is not hopeless. It’s going to be a hell of a lot harder, but not hopeless. Don’t give up.
Every day we encounter people who have to cross the street or physically lift their dog when we walk by. Or my least favorite command: “leave it, leave it, leave it…”. It’s sad. Now your just addressing a symptom, not the problem.
I’m very proud of my pit bull when she can calmly pass by a hysterical barking dog. The other person should be ashamed, and not of their dog: of themselves, because it’s not the dogs fault.
A lot of people in cities like San Francisco treat dogs like house cats, only taking them outside to 1 and 2. This is cruel. Dogs are social animals. Some may need social interaction more than others, but I would argue that socialization is even more necessary for dogs that appear aloof to other dogs. At some point you will see the result of lack of socialization, and by that point it could be bad. Many times I have seen a dog snap at another dog or person only to have their guardian exclaim “I don’t know what happened! He’s never done that before!” Of course dogs have misunderstandings or just don’t like one another, just like humans, but 90% of the time a dog misinterpreted another dogs signals because it has not learned to read the language of dogs.