John McCain harbors a lot of bitterness, and his grudges run silent but deep. He hasn't gotten over not being elected President and never will. Neither will Jeb Bush, Al Gore or John Kerry. They all will go to their graves convinced that there was a grave injustice served and all have gone or are going sundry varieties of crazy. Al Gore has been the most visibly unhinged but I believe McCain to be literally driven nuts over it, not that he was the model of mental stability before. Some believe carrying around such bitterness and toxicity has health consequences. It's certainly not helpful with treatment and recovery from a cancer diagnosis.
McCain may be more bitter about losing to Bush in 2000 than to Obama in 2008.
Anyway, I guess the question now is to find a losing candidate who wasn't bitter.
Maybe somebody who never expected to win: John W. Davis or Alton E. Parker.