Why Do We Replay Games?

Game Informer Magazine has kicked up a new three-page feature that poses the question of why we replay certain games multiple times over. For me, it's usually about the nostalgia:
When you feel exhausted just thinking about starting an RPG boasting (Epic adventure delivering over 80 hours of gameplay complete with a vast array of power-ups, advanced attacks and over 100 different skills,) replaying a game from 1985 might be for you. Take some time to detoxify from all the unadulterated trash (thanks, Nolan) that floods the current market and replay some classics from the past few decades.

Sometimes for the co-op:
Co-op play provides another motive to replay a game by taking a friend along for the ride. After beating the single-player campaign mode in Rainbow Six Vegas 2, you and friend can join forces and play through again a standard element in many FPS titles. Playing co-op brings a whole new element to the mix since it is no longer only your hide on the line. Once the two of you have mastered the game together, it may be time to introduce another element to the mix online multiplayer action.

And, every so often, out of pure addiction:
Does it count as replaying a game if you never really stopped in the first place? I think we all have at least one friend who suffers from a World of Warcraft addiction. Just like multiplayer online matches, MMOs are dynamic by nature. While you may choose to spend all your time leveling up one character and going on raids with friends you may also have a portfolio of characters of different races ready to play on a whim. Granted, each gameplay experience may be drastically different, but in theory these games are designed to be (replayed) an infinite amount of times.

Now you'll have to excuse me so I can continue my journeys as the Nameless One.