I think one of the reasons we are seeing mass shootings increase in the USA is because these people see that they can do it, from a cultural view point. This is hard for me to explain, so I apologize in advance.
Like Bishop said before, it was super common for teenagers and young adults to carry pocket knives, even guns with them in everyday life. Hell, I live in a rural area, and many used to come to school with their rifle or shotgun in their pickup on the rack so they could hunt before school and after. Culturally here, it was acceptable to have guns, BUT also to know proper safety.
Fast forward to today. With the lack of mental health funding, access to guns, access to disturbing web pages/social media/etc., violent games (which I do enjoy), and the, what I believe, TOTAL lack of cultural empathy towards our neighbors, we are seeing young people (mostly male) act out in these aggressive mass shootings. Lax touched upon individualism in a previous post, and I agree with him. We are all so sheltered from the outside world, so protected from consequences as well, that we are no longer living within communities, but rather almost a VR where we are merely a character.