I think there's a genuine lack of empathy for strangers (See: Dunbar's Number), which is especially prevalent among the young. I find that as I, at least, get older, I start to feel a bit more for people getting hurt.
Unless they're idiots doing it to themselves (Grape Stomping Lady). Then that shit's just funny.
Personal stupidity in the situation goes a long way, as does the knowledge (inferred or otherwise) that everyone walked (or at least hobbled) away from the scene in question.
I'd be lying if I didn't chuckle madly at all those monthly fail videos, of course. I will say it's usually more amusing if the people in question are younger (especially teens. Nothing beats watching a teenager do something stupid and get some sort of easily-survivable-yet-painful injury because of it. That'll learn that youth to be young).