The answer is self-pity, according to Jeffrey Toobin. The New Yorker published his review of Thomas's memoir, My Grandfather's Son, and it's the best I've seen. Toobin knows enough to be able to point out the inconsistencies of Thomas' wrath, and that makes the review particularly fun. The historical context Toobin provides is also good.
You can listen to a 12.5 minute interview with Toobin here about Thomas and Toobin's book The Nine.