Reason Number 9: After 17 years, still pissed that Leno got Tonight Show gig and he didn't.
Reason Number 6: When Letterman calls his mother, she always answers the phone, "David who?! Then hangs up.
Reason Number 5: Finds it perfectly acceptable that it took him five years before he finally decided to marry the mother of his child.
Reason Number 4: His idea of socializing is to stand in front of a mirror for hours on end just admiring his reflection.
Reason Number 3: Only person in the universe who doesn't think his mimicking sidekick Paul Shaffer doesn't look like Warren T. Rat in the animated movie An American Tail.
Reason Number 2: Has a plaque on his desk that says. "If It Weren't For Me The Sun Would Have No Reason To Shine!"
And The Number One Reason David Letterman Is A Jerk: He's An Egomaniacal Ass!!