Sad but true - particularly so in the last couple of years.
Perhaps he is simply bored out of his mind? What is left of his mind anyway...
For one thing he never got over being passed over for Johnny Carson's chair. Then when Leno started winning the late night ratings consistently, it seemed he started getting desperate, trying to get his writers to be more "edgy."
Then when Bush became POTUS, they all thought they had themselves a cash cow, and for Letterman at least he went to that well about a million times too often while Leno somehow found better things to joke about and never was hateful, delivering actually funny material night after night, and continued to kick Letterman's ass night after night.
Letterman is a bitter, angry rich old codger. When Conan starts kicking his ass as well, he'll probably just hang it up. As he should.