Surprised nobody is mentioning climate change. Young people are supposed to be contributing and centering their lives around a kind of societal structure which is increasingly seen as doomed. The constant acceleration of capital is bumping up against the material finitude of an increasingly depleted Earth.
Conservatives have a vision of the doomed future, built around walls and hoarding resources against impending scarcity. But liberals don't really have a good answer outside of "bring the super-rich to heel and use their resources to invest in mitigations," a view which is anathema to politicians who depend on the super-rich to continue holding power.
People are depressed because there doesn't seem to be a future.