Owner's training their dogs not to be aggressive or overly protective is a great idea - in theory. That will likely work for most breeds - but not all. No one can "train out" thousands of years of breeding for specific characteristics. This recent Time Magazine article illustrates my point:
Pit bulls make up only 6% of the dog population, but they’re responsible for 68% of dog attacks and 52% of dog-related deaths since 1982, according to research compiled by Merritt Clifton, editor of Animals 24-7, an animal-news organization that focuses on humane work and animal-cruelty prevention.
Another report published in the April 2011 issue of Annals of Surgery found that one person is killed by a pit bull every 14 days, two people are injured by a pit bull every day, and young children are especially at risk.
...critics say that pit bulls are inherently dangerous no matter how they’re treated, because violence is in their DNA. “Why do herding dogs herd? Why do pointing dogs point? They don’t learn that behavior, that’s selective behavior,” says Colleen Lynn, president and founder of DogsBite.org, a national dog-bite-victims group dedicated to reducing dog attacks. “Pit bulls were specifically bred to go into that pit with incredible aggression and fight.”
http://time.com/2891180/kfc-and-the-pit-bull-attack-of-a-little-girl/