Quality of parents/teachers/mentors, availability of guns, violence on TV/video games, the internet, social media, bullying, cyber-bullying, child abuse, prescription drugs, non-prescription drugs, the 24/7 news cycle, mental illness/psychosis/sociopathy, perceived lack of opportunity, etc...surely I missed some.
But, overall, violent crime is way down from the 70s, 80s and 90s. But the perception is that it is ever increasing because of the 24/7 news cycle. Also, an interesting podcast I listened to recently talked about abortion and a reduction in unwantenness is the cause for the reduction in violent crime.
Also, and I don't want to sound cold, but you're far more likely to die from someone texting while driving than in a mass shooting. The 4,637 people who died as a result of texting and driving last year are hardly reported on.
It's a symptom more than a problem on the grand scale, IMO.