Many dogs have certain characteristics bred into them. These chararcteristics are instinctual to them.
Jack Russell terriers for instance (of which I own one). He has never been around other terriers but, he just naturally hunts and kills (or tries to) other animals. Mice, cats, squirrels, groundhogs, raccoons (but he loves kids, which is kind of unusual for a Jack).
Rottweilers (another which I have owned) tend to be very territorial and protectective. If another animal or person comes into to their yard uninvited they could be in for some trouble. Mine crushed a Chihuahua which dug it's way into our yard, scared off a cop who was looking over our wall to see if a prowler had exited through our yard, "Nope! He didn't go that way! But, if we had friends or kids over they could roughhouse with her or sit on her and she was gentle as a lamb.
My mother and stepdad had an Australian Sheppard mix puppy which was about 4 months old which with no training at all just naturally herded their dairy cattle. It was cute seeing that little 15 or 20 lb puppy running around yapping at the heels of those cows.
Pitbulls were bred to fight. It just comes natural to them. Not saying they can't be wonderful pets or that they should be legislated against. Just that they have a tendency to want to fight and people really should take that into account before getting one.