According to "professor" Adam Lankford it's because we have too much freedom.
American exceptionalism and the ‘exceptionally American’ problem of mass shootings
The "false promise" of the American Dream? What "promise"? See, this is where the pinko, commie, tree hugging, dope smoking liberals have taken this country. There is no "promise" of success in America and there never has been. The only promise was that everyone would have the opportunity to seek success on their own terms and in accordance with the efforts and abilities they brought to the table. But that wasn't good enough for the Progressives starting way back with Teddy Roosevelt.
The idea that there is supposed to be some great American "promise" of success started as a purely political construction for the sole purpose of garnering votes. Little by little, though, those in elected offices figured that if they could legislate a little "success" into the mix then those votes would keep on coming and sure as hell, that's where we are now. We hear it day in and day out now. Everything will be fine and dandy if we let government do this for us or if we force them to do that for us. Well guess what, there is no freaking Utopia!! It doesn't exist and never will exist!! If you want to live in the land of milk and honey you have one option.....BUILD IT YOURSELF!!!
Maybe this jackass "professor" does kind of have a point though. Maybe part of the problem really is that people in this country aren't feeling fulfilled. That could well be part of the problem. But, hey, when you abandon God and seek comfort in the arms of the harlot that is government you're pretty well guaranteeing that feeling of emptiness that follows the realization that you just spent your rent money on lipstick, pancake makeup and a case of the clap.