There are always going to be new MMOs coming to the market, promising so much more than you're currently getting and trying to take your business over to their camp. Perhaps you like what you see and hear so much that you cancel your old game accounts, possibly delete their clients and devote yourself to the new MMO. Things go okay for a while, but then you find yourself missing things about that old game -- maybe the community, your former characters, the gameplay, the content -- that make you want to go back. It's your classic "grass is greener" scenario and we hear about it all the time.
That's not always how it goes though. It doesn't have to be a quick process. Maybe you quit a game many years ago and then suddenly out of nowhere, someone brings it up and (perhaps due to being bored with newer offerings) you just have to see how that world is going. It's a lot like an old partner, in that there's often a comfortable familiarity with a game you've spent a lot of time playing that you just can't get anywhere else.
Have you had this happen to you? Is there one particular game that, no matter how many others you try, you just keep coming back to? Tell us about it in the comments section.