Dog breeds

I saw a question on here last night asking what you favorite dog is. I am not really curious on what you fav dog is, but more curious as to why everyone thinks that certain breeds are aggressive and dangerous.

I have a rottwieler that is the sweetest dog ever. Loves kids with all her heart and is a major people person. Don't get me wrong, she is protective when she needs to be but not aggressive at all. I have a friend who has a pit bull and he is like a puppy. Always wanting to play and loves everyone. It really bugs me that people pass judgement so quickly on dogs just cuz of the breed.