Humans used artificial selection to create different dog breeds with different skills. Some of those breeds were bred for aggressive traits, whether that was for fighting other dogs, protecting sheep against wolves and bears, or literally joining men in going out to war. Acknowledging this fact is not saying all bull breeds are bad, just that you need to respect the breed and realize some of these dogs will likely be higher in aggressive traits. A dog that was bred to fight other dogs might do things that surprise the owner, especially because it acts friendly and sweet within it's own "pack", you might think that it will be sweet with other dogs or kids. This isn't always the case. It's very very important people know what they're getting into.