I was recently on a Toronto streetcar and witnessed an altercation between two passengers. I didn't see how it started, what I saw was the result; the blow-up, the anger, the confrontation.
One party exited the streetcar at the next stop and the altercation was over and done with. No one hurt, no big deal. Just another one of the many crazy things you witness on a daily basis living in a big city!
But what I noticed after a few minutes is that I hadn't let it go. My wheels were spinning, thoughts going like crazy about what I would have done if things had escalated, how I would have handled the situation if I was in one of the participants shoes, how something like this could happen to me in the future. I wasn't even involved, yet was totally wound up.
It's important to note that this isn't unusual. We tend to take these things on. Because we as humans are gifted with the ability to imagine and project, we can really run with these sorts of scenarios. Contrast this with animals who can witness one of their herd get taken down by a predator one minute and be snacking on grass in the next, totally unfazed.
Because of this unique human attribute, we need unique ways to deal with it. This is why we're blessed with the vagus nerve.