I don't know what it is, there's lots of things that are probably partially to blame. And guns aren't on the list. A generation or two ago, it was common for high schools to have official school-sponsored rifle teams. Rifles could be seen hanging in the back window of pickup trucks. They were ubiquitous and easier to obtain than now. But there weren't mass shootings in any sort of frequency.
What changed?
There's certainly the role of psychotropic medications that are doled out like candy. But the meds are more the symptom than the problem themselves. Why are so many young boys put on mind altering prescription medications? If you look at the reasons given, most of them could be summarized as "for behaving like young boys". Males in this culture are not allowed to behave in normal masculine ways anymore. From preschool they are told their normal inclinations are aberrant, disruptive, in need of modification. Do we really know the long term effects of these medications, particularly when given to children whose brains are still developing? Medical science doesn't even really know the mechanism of action for many of these drugs.
I think that is part of it, this War on Maleness and the chemical weapons used to fight it.