I'm sure that each time this happens, we all sit down and just try to figure out how the hell does this stuff happen. And maybe because most of us have never lived outside the US we really can't see why this seems to happen here and nowhere else. I've been to Europe and I've lived there, part of my childhood was spent there and honestly growing up in America as a kid is fucking torture.
First of all I guess you have to realize that an act like this is not something that a sane person does. School shootings are the result of delusional state of mind resulting from anyone of hundreds of mental disorders. Once you know what the problem is you then can look at the possible causes. But lets look at a typical childhood in the US living in an urban or metropolitan area. Kids need space, they need a place to go where they can be away from adults(at least at a certain age) to do what they want to do. Soccer, Tennis and Basketball may fulfill the needs of some children but some kids want to Dirtbike, Mountainbike, Blow Shit Up,... and none of these activities can be done inside any city, which means they need a car and since they can't drive they rely solely on their parents who work ALL day and never have time for the kids. So kids are stuck in the city so they have to worry about Cops, if I ride my bike I need a helmet or I get a ticket, and then I have to go to traffic school and my parents will yell at me and the Cop will scare me with a big lecture. If I go to the park all I can do is go on the swing or play Tennis, Basketball, Soccer and even then I have to worry about breaking a window with a ball. Anywhere kids go its private property and constantly have to worry about trespassing and all these laws/rules/regulations that kids shouldn't have to worry about. And kids are SUPPOSED to do stupid shit, its part of growing up, you fuck up when you're a kid so that when you grown up you don't screw up when it counts. Lately we've been locking up all the kids that screw up as kids treating them like they did some unspeakable act and that they should know better when there is no way they could. Then if a kid gets into trouble they then have to deal with all the stress of court, the possibility of going to jail... On top of that going to school as a kid for most is a great experience but its not like that for everyone, for some it is pure hell and rather than building a society where we all help each other out and we all look out for each other we instead have a competitive kill or be killed environment is schools. The stress is driving these kids crazy and who can blame them.
I Europe at least where I lived everything was more open. There was lots of empty land where you didn't even think if somebody owned it or not, it was just some forest or some open field, you didn't do damage on purpose but you had fun, there was always lots of room and you never had to worry about someone suing you or getting thrown into jail. If someone wanted you to leave they told you to go and you did and that was that. If you broke something you settled it with the owner and your parents. People there didn't call the cops and the SWAT team first and then talk to you, and they didn't call their lawyers for every little problem. The cops were reasonable and although they could drag a kid to jail, they never did, they always took you home to your parents and let them deal with you. When you played on the street you didn't have to worry about breaking a window or the cops or the neighbor that just sits and waits for something to happen so that he can call the cops. You just feel more free and less stressed out because you don't have to worry about so much crap.