It's probably a mix, but I'd wager it's primarily a side effect of the existence of the internet. Before you'd hear the bad things in your immediate community, the really bad things from your country/region/whatever, and really only the truly horrible stuff from the world as a whole. Now you can have a constant feed of every bad thing that happens everywhere. As you see more of it, you're bound to get desensitized.
But hey on the otherhand I have unlimited videos of cute puppies and I'd be lying if I said that wasn't a pretty big upside.