I think ultimately it is the media. Mix crazy people who have been rejected their entire lives that want desperately to get famous, and they see an easy way to do it. CNN even created a leaderboard on one of their shows the other day, showing who's killed the most in a mass shooting.
Blaming the media for shootings is like the conservative equivalent of liberals blaming guns for mass shootings. That's not to say that the media doesn't have some responsibility and that the ease of obtaining guns by whackos doesn't play a factor. However, it's such a complex problem, and the need for each side of the political divide to assign blame so simply only makes the problem worse by not allowing us to really try to develop some real solutions. Politicians, the media, and all of us spend more time bickering over nonsense and trying to blame the policies of the other side. These events happen and everybody just runs to their corners. It's sad, because that's the worst way to solve the problem.