Humans spot risk, oddities in recognisable patterns and are far more likely to remember negative events because they are hard wired to do so.
As hunter-gatherers (you know, in the time before writing and the invention of religion) we'd either learn to spot danger and do what we can to avoid it or have decreased chances of survival...fast forward 100(?) thousands years or so and the information revolution gave us access to numerous sources of negativity and percieved risk. Once upon a time to find a heretic you had to travel! -now you can speak with a blasphemer in under 10 seconds just go to a forum or heck, skype them!
Some people are unable to detach, disassociate or become desensitised. Often the stress is not even about real threats just amplified mass fear of awful weather, cheating in their favourite sport, injustice of what they consider their basic rights etc.
Basically like all humans, people suffer from the human condition. Humans are irrational. They are more concerned with controlling borders than traffic accidents despite traffic being the proven killer. They are more worried about terrorists than the flu and yet one kils hundreds of times more. Humans are scared their children might develop diametrically opposed beliefs if they associate with certain other kids but turn a blind eye to the negative imapcts of their own beliefs because they seldom scrutinize it...the list goes on.
Americans are not crazy but they are being driven crazy by a political system that preys on fear. News network that compete to report the latest disaster. Corporations that research their insecurities about their body and sell them shit they do not need.
Actually this happens everywhere...somehow this more pronounced in the US for reasons I'll let others suggest.