My hyphothesis as to one of the major reasons that young people are unhappy in the US: meanness is discouraging and demoralizing, kindness and caring about other people breeds contentment. And we are at a point in our cultural cycle that fucking elevates and worships meanness, cruelty, selfishness, and hurting others, and demeans and diminishes ANY concept of the common good, doing things for the benefit of your fellow man, and the like (hell, see cum rocket's reply above). You think that young people don't see that? They see it clear as day.
Contrast that with one of the countries highest on the happiness index, Finland (7 for youth, 2 for elderly). The observation hit my wife like a hammer when we visited last year: they care about each other, they care about their children. As in, it is a point of national pride and identity. It is conspicuous, it is woven into their daily lives and the stories they tell each other. And I thought there was an interesting point in the discussion of "happiness" as it is observed in Finland. It's not about walking around giggling with a shit-eating grin on your face; it's about contentment, and being satisfied.
Culturally, we don't value "having enough" at all. You must either have excess -- wild success, conspicuous cars and fashion, etc. -- or you are a loser and a failure. In someplace like Finland, happiness/contentment for the bulk of the country is being able to spend 2 weeks in the summer at a cabin on a lake. Many of them don't even have electricity. You sauna, you plunge in the cold lake, you drink by the fire with friends, you eat some food that makes the Finns happy (but man, most folks wouldn't be thrilled with it). And it makes you happy. Here? You best be blowing it out on a tropical beach or somesuch, eating $200 per person dinners, or else you suck. Yes, there are outliers (there are rich Finns, there are Americans who are happy and satisfied with ordinary lives). But our dominant cultural theme is NOT healthy.
Kindness. Taking care of each other. Finding joy in simple things. Finding happiness in connection, not in materiality. Those are the simple and not-exactly-secret keys to happiness. Our culture is far away from all of those things, and running further away from them every day. We should stop running that direction. Then, we should take some steps in the OTHER direction.