Oh golly...lets look at a number of factors.
-For the last 30 years more and more people have been diagnosed with mental health issues and been prescribed numerous cocktails of medications. A significant portion of the country literally cant function without chemical assistance.
-We have raised generations of kids on violent video games. No...I'm not blaming the video games for the shootings but those ideas tend to fester in peoples brains.
-Increasing numbers of people have been indoctrinated with the 'fairness' doctrine that teaches them that the world is supposed to be a safe place and people arent supposed to hurt their feelings and subsequently they shouldnt have to deal with situations that they find hurtful or damaging to their poor little psyche.
-We have stopped preparing people for life and the real world which means sometime the job sucks, sometimes the girl breaks your heart, sometimes you arent going to like people and they arent going to like you. We stopped teaching people about personal responsibility and owning their own lives and their own shitty choices.
If you really want an answer to the question, look at each of the cases and read for yourself and see if you can see the themes.
The full data set from our in-depth investigation into mass shootings.
www.motherjones.com