Happiness is more complex than your comment would make it seem. There is no absolute bar you can pass after which you habe to be happy. Happieness is fundamentally relative, since happiness is the gap between where you want to be and where you are.

So one part of this generation being unhappy is thst their life on average got objectively harder than those of their parents. Who hasn't heard the story of a boomer who as a 20-something bought a house from the money made in a job that wouldn't even pay rent these days.

But that isn't all, since happiness is relative the youth today sees a fictional image of what they are supposed to live like every day in the internet and most of them are nowhere close to that. So it both became objectibely harder and the bar moved up at the same time, so if more people whine, it is because they have reason to.

I don't say life wasn't hard in the past decades, but people had the sense that if they worked hard, they could potentially reach a state that felt good to them. This is less true today. Even in my generation (Millenials) many have given up even considering the image of retirement, because our retirement is a value that we know will be sacrificed to the capitalist gods, like the whole damn planet.