Old man fight
Guy hit with bat
Alien wake-up prank goes bad
Guy Gets kicked in balls
Those are some funny videos. No one can deny that. Some of them you may have seen before, some not. Doesn't matter. If you don't laugh at the point of impact, then you do once you see the guy writhing in pain. As long as he is not dead or paralyzed, we laugh. Sometimes, we even laugh then. But why?
As it turns out, we aren't all just rotten fuckers who derive pleasure from the misfortune of others. Or, actually, we are. But it is not our fault. It is the way our brain functions.
Science even has a term for it. "Schadenfreude" It is a German word that I swear doesn't mean "haha jew roast!" Instead, it has something to do with 'enjoying the trivial suffering of others'. Apparently, the Germans aren't the only ones to have a word for it. Just about every single language humans speak has a term to describe what we on the internet refer to as 'lol pwnzd'.
So why doesn't your girlfriend laugh when you get drunk and kick her brother in the balls? Not surprisingly, it is a decidedly male trait.
Our brains have what is called an 'empathy center'. During several studies, scientists have found that this section, which some of those very same scientists believe is in the parietal cortex part of the brain, lights up during certain sets of circumstances. The studies have found that this part of the brain lights up when we see what we perceive as 'bad' people get hurt. However, in men, and not women, it also lights up when we see anyone get hurt, regardless of whether they are bad, good, or some dude we don't even know.
It may also have something to do with envy as well. Those very same studies, that I keep quoting but won't link to because you have Google you fuckers, show that there might very well be a correlation between the misfortune that we have suffered ourselves and just wanting to see others go through it too. Especially when we subconsciencly see those people as more successful than we are. In other words, screw those guys for having more money/better cars/hotter girlfriends than we do, let them get kicked in the balls for our amusement.
This makes sense. I mean, how many of us laugh when a homeless man takes a unprovoked shot from a rich pompous bastard? Not many, most of us wish the ill will on the perpetrator, seeing as how we don't see the homeless man as having something we wish we did. Unless we see ourselves as worse off than the homeless man, and in that case it is damn funny.
This also explains why we don't like to see animals get hurt, but a human doesn't bother us. We see animals as innocent creatures undeserving of spontaneous violence. Humans, however, as we explained earlier, are by nature rotten bastards who deserve everything they get. Unless... of course, they are less well off than we are. At least the male ones anyways.
To sum this all up in one simple sentence: We laugh because that is how evolution made us.