Perhaps, because we are programmed to enable our survival by noticing the negative things. If there is a beautiful flower and an angry dog, we want to notice the dog first.
Or perhaps because we listen to a daily dose of news media that plays to sensationalism in order to sell subscriptions.
Or perhaps because, our visual media including movies, television, video games in filled with never ending violence that we keep getting looped.
Or perhaps it is because we don't know how easy it is to change things.
No matter what the reason, if we can change our reaction to a stimulus then we can create new possibilities. If we change our reactions often enough, then we rewire our brain to do it without thinking and then we each can create a different kind of world. I do believe that this can be done, one reaction at a time. I sound like a sound byte for the movie and it is a movie worth noticing for just this kind of message.