Pain.

Pain, what can you say? It hurts when it happens to you, but when it happens to others its funny. Of course you don't laugh right away... You make sure that they're okay and in no real danger. If they're okay than its funny. Its funny because they tripped and smacked the wall, fell down the stairs, or hit their head on something. Yes, it could be detrimental to them and they could be in possible danger, but since its happened to you before you know that it sucks to them and so it's funny. You know you shouldn't laugh, but you just can't help it. That's why next time someone gets hurt make sure they're okay then enjoy it, because it's going to happen to you sooner or later.