My first MMO was Runescape 2 and then WoW.

Before that I played Sims 2 (and a few other games of different genres) because I liked improving my status/power, leveling my skills and gaining money in a game. I also enjoyed creating and tweaking characters. But sometimes it was lonely. MMOs offer all of that and more: co-operation, exploration, adventures, fantasy and challenge. I've not played Sims since and almost never play other games than MMOs, especially not single player. They're boring to me.