My mom and sister have chihuahuas and my grandfather has a shih tzu, they are all super sweet dogs who have ever been aggressive towards anyone. Meanwhile, a family that lives down the street from my grandfather had a golden retriever that had to be put down because it kept attacking kids unprovoked. Sure, some dogs are more predisposed to be aggressive than others but the way they are raised and the environment they live in plays a massive role in how a dog acts.