[H] Why is multiplayer so addictive?

[Veech]

Last of the three Orange Box games that I tried was Team Fortress 2.  I had 
never gotten into online gaming but I loved Portal and Ep 2 of HL2 so I 
though if TF2 was half as good it would be worth investigating.  Damn, it's 
like crack!  I really am spending a lot of time right now, especially 
playing the Dustbowl map.  Not to the point of quitting my job or anything, 
but it takes up almost all of my spare time.

What makes this stuff so addictive?