I actually think that viewing this through the lens of just mass shootings is probably missing the point. I think mass shootings are basically just a symptom of something that is affecting men in general in this country. Men are basically doing worse than before. There are a lot of reasons and things to discuss about that, but at the end of the day males drop out of school at a higher rate, they kill themselves at a higher rate, they abuse their spouses at a higher rate, they are incarcerated at a higher rate, they kill at a higher rate. It's weird to think about considering the male position in society but I strongly believe we as a society are failing boys right now, and globalization is accelerating the problem because low-skilled manual labor jobs that used to be enough to support a family are disappearing and we have a lot of guys with no real prospects because of it. Those jobs used to not only be income but social support structures as well. And we've seen more and more that as males get older, the biggest issue we have is a lack of friends.
I'm not saying I have the answers, but since we're on a message board and we're all talking out of our asses sometimes I'll give my opinion anyway. I believe it is due to the lack of male influence in pre-adolescent and adolescent life. Young boys grow up being taught by practically only women through elementary school, and they might have a few more men teaching them in high school. Not having men around them in this environment leaves them without teachers they can relate to on male-specific issues. It also leaves them to only have role models that are basically inaccessible and idealized -- your LeBrons, your Obamas or Trumps, your PewDiePies. But they can't interact with these people, they can't really grow from them. So boys are left in a very isolated world where they can't learn how to effectively be men FROM men. I think it's really undervalued how important it is to have that kind of model for yourself. It's important to have somebody you can relate to, to talk to "man to man" about things.