Huh. It's mentioned that he was inspired by a study showing that ~$70k is the 'optimal' happiness point.
I think I've read that study. The best way I can explain it is that it turns out that money doesn't buy happiness, but it sure can be used to prevent or 'pay off' unhappiness.
IE, when you're hitting ~$70k/year, you stop gathering extra unhappiness from lack of funds.
Extra money will make you happier if you're unhappy because you're having trouble meeting your bills, having to budget every dollar, etc... If you're unhappy because you have no friends, or don't have a girlfriend or something, then more money will only provide a transitional, almost illusionary gain in happiness.