It is their nature.................
Why do border collies and other herding breeds herd livestock? It is what they have been bred to do.
And it's not just the owners as many will claim. It's frequently the "sweetest dog I ever knew", a family pet that has been loved and well cared for since it was a pup. I've seen it myself with my Aunt's pit and another friend's pit. They are "great family dogs" until suddenly they are not. When my daughter was a child, there was no doubt in anyone presents mind that the sweet little pitbull would have killed her if allowed. There was no teasing or taunting. It was the first time the dog ever saw her and it was like a switch was flipped. The vast majority of dog bites are a warning to get back, leave my toy/food/puppies alone, leaving a couple puncture wounds. Pits are not warning when they bite, they are trying to kill.