Okay, where was I?
Violence. Why do we have such a large tolerance for it, some people ask? Well, those people would at very least seem to be trapped in the present, with no sense of history. For most of human history, violence has been the norm rather than an exception. Parents beat children bloody, men were not allowed to beat their wives with a stick wider than their thumbs, police beat anybody they could find, wars were fought close enough to home that people could and did drive out to watch.
But the root causes. Ah. The why.
Well, let's go back to our origins and speculate widely.
Let's say that you're a pre-human primate. What does violence mean to you? Well, if you see a neighbor torn apart by a pack of baboons, it means that you are still alive. If you see your friends tear apart a small deer, it means that you are going to eat. If you and your friends kill the neighboring tribe, it means you will survive.
If seeing violence makes you sick, you probably won't survive. If you won't club another animal to death to protect your babies, your line will probably die out. In a world without lawsuits, disputes are settled in one of two ways: one side bluffs the other out or one side beats the other side physically.
Those who tolerated or even liked violence survived. And passed their genes down to their descendants. Us.