Why Is Killing Zombies So Fun?

I don't recall if I mentioned this before, but we recently got the game Call of Duty: Black Ops. The multiplayer is fun and all, but what we really love to play is the Zombie mode. It's pretty difficult for us noobs, but we really enjoy it! We've spent many nights up till the break of dawn playing this game. I believe that farthest we've ever gotten so far has been Round 10. That's pretty good for us!
Well, I think I'm going to play this game now. Talking about it makes me want to play!!