It's a bit in the style of corporate self-help books so I can't bring myself to read more than a few pages at a time (i.e., I am keeping it in the bathroom right now), but I'm about halfway through and the author makes the point that willpower is like a muscle - if we exercise it too much we fail at a certain point. Philandering politicians were a case in point - if they resist the temptation to say what they really believe all day, and pretend to be someone they're not to get votes, at a certain point they have no willpower left for their personal lives.