Why Are We So Unhappy When We Have So Much?

Why are we Americans so unhappy when we have so much? I don’t mean to belittle the difficult and genuine problems people face. Anyone involved in helping people knows that there are some real horror stories out there.
But as a nation, we enjoy more of the factors that ought to go toward making life happy than any other nation in history: longer average lifespans, good medical care, plenty of food, extra clothing, nice homes, cars, and all the gadgets of American life. So why so many unhappy people?