Team Fortress 2 are just flirtations, MMOs are full-on relationships. You might look with vague interest at EVE Online even as you briefly date Star Trek Online. Perhaps you broke up with Final Fantasy XI to get together with Aion. Or maybe you and EverQuest are celebrating another anniversary and wondering why people feel the need to overextend a relationship metaphor.
Joking aside, MMOs require a larger time investment than any single-player romp. While claims of playing since a game's beta are almost a self-parody, there's a lot to be said for dedication to a single game, and it breeds far more resources for a player to make use of. On the flip side, of course, you can only spend time in so many different worlds, and having a breadth of different experiences is more to the tastes of some players.
So what MMO have you been playing the longest? Is it a game where your subscription has never lapsed, or one you've frequently left and come back to? Are you happy to keep playing it, or is it that crazy one that you keep almost breaking up with but never quite go the distance?