Because progress and growth makes us wealthier and happier? It's pretty simple.

People say "Oh, but GDP isn't everything" - but it's correlated with almost everything good, so might as well be.

GDP is correlated only while good things are increasing - forcing every married family to divorce at gunpoint and become two family households would greatly increase GDP - but I don't think we'd agree that's good.

This. The prospect of a brighter future at least means capital and labor are fighting for slices of a bigger pie. If the pie per capita stays constant or shrinks there will be a lot more anti-social behavior to response to the zero-sum environment.