I think people learn to rationalize bad behavior for the sake of something they percieve to be greater than themselves. Bill Clinton probably told himself if I don't lie I will weaken my parties chances...I must lie for their sake. I don't believe GB, GHWB, or BO lied I believe they went back on their word. Which to me is a whole different animal. I think they went back on their word for the same reasons though. "This thing (Iraq, middle class tax hikes,etc...) is greater and more important than my promise. I must do it for the sake of the greater good."