How Did George Carlin Stay So Bitter?

George Carlin built his comedic-career on making his stand-up delivery bitter and angry.  I think once a comic reaches the pinnacle of success, like George Carlin eventually did, you'd move on from being angry, and reach a plateau of satisfaction.  I can kind of understand the bitterness George Carlin displays early in his career, because he had to struggle to achieve fame.  George Carlin had to work extremely hard to continue his angst, or maybe he was just angry by nature.  His angst comes off as very contrived when I watch his later stand-ups.