That Joe Lieberman is a small and petty man, and has allowed imagined personal slights to be a springboard to vindictive behavior:

So why is he doing this? Because he’s bitter. According to former staffers and associates, he was upset by his dismal showing in the 2004 Democratic presidential primary. And he was enraged by the tepid support he got from many party leaders in 2006, when he lost the Democratic primary to an anti-war activist and won reelection as an independent. Gradually, this personal alienation has eaten away at his liberal domestic views. His staff has grown markedly more conservative in recent years, and his closest friends in Congress are now Republicans John McCain and Lindsey Graham. For Lieberman, the personal has become political, and it has pushed him further to the right.

Simply put, Joe Lieberman is a vile, vain, vindictive, and petty person. and because he feels that he was not sufficiently worshipped by the Democratuc Party, he is determined to extract his revenge.

What a petty and pathetic excuse for a human being.