Because it's wierd.
I mean, when you punch somebody in the face, you have suddenly inflicted terrible pain on that person. Meanwhile, your emotions are those of twisted pride and sadistic enjoyment. Why is it that we don't feel the pain too?
Hurting others is a primitive test of telepathy. We want to enter a personn's mind. We want to understand why we can't all share this world in peace. We want to find that breaking point when all the horror of human psychic suffering finally comes to a point where we can understand each other truly.
But seriously, isn't it wierd when some poor sap is moaning and groaning on the ground. It's freaking wierd. Because you feel fine and he feels horrible. There's some hidden emotion that is triggered by this absurd observation. And lot's of people are so curious about it that they forcefully recreate the situation over and over again until it makes sense.
Or else it's because they get that wonderful feeling when they hurt you. You know, the kind of feeling you get when you personally beat down 3 people in a Starcraft FFA.