He's lonely and he's always getting picked last in gym class:
JOE KLEIN: This is just a guess, but it's an educated and a reported guess. The Democrats in the Senate are getting really, really angry at Joe Lieberman, especially because he's been accusing them of undermining the troops' morale. And Joe Lieberman isn't too happy with the Democrats, either. I think there's going to be an explosion and perhaps a party switch pretty soon.
The only issue that separates Lieberman from the rest of the Democrats is the Iraq war, and specifically the surge. Yes, he's an Iran hawk, but so are a lot of Democrats. Yes, he's not a down-the-line liberal on social issues, but neither is around 1/3 of the party in Congress. He'd really be willing to quit his party because they're wrong on a war which he thinks will be over by the end of this year? There's more pragmatism on an average episode of My Super Sweet 16. Good call, 50 percent of Connecticut!
More pivotal reason why this switch won't happen: If Lieberman becomes a Republican no one will care about him anymore. A pro-war liberal Democrat is a media curio and a useful guy to have on your side when you're promoting a new resolution or a speech at AEI. A pro-war liberal Republican is… George Voinovich. Stay ornery, don't switch parties, and people will care about you. This is the trick Zell Miller mastered, and Chuck Hagel has figured out pretty well, for all of his absolute uselessness.