LAN parties are the source of some of the bestest multiplayer gaming ever (and I'm all about multiplayer). There are few things more rewarding than being able to shoot someone on your computer and hear, from a room down the hall, "Aw, what the fuck?!" Doubly so if you can hear the pounding of mouse-on-mousepad frustration.
Okay... so maybe I enjoy my friend's pain a little too much.