Aggressive behavior in dogs is usually an indication that the dog is scared, worried, or not getting enough to eat or play with.
When puppies exhibit aggressive behaviors like resource guarding, reactivity,
or growling and biting, it's usually because they're still young and need to adjust to their new environment and their own minds.
But having working with dogs for twenty years, I can tell with confidence that some breeds are more prone than others to violent behavior.
I hesitate to label these as "aggressive dog breeds," but it's clear that
every one of these dogs needs a seasoned handler and the right
kind of socialization and training to develop a well-rounded character.