I imagine there's a high correlation to quality of life of the middle class, and whether their government is functionally competent and able to get public works done (building roads, rails, bridges).

I'm pretty well convinced that the US' days of getting anything done are past us. We had the perfect opportunity to put people to work on new infrastructure while the world was shut down and completely squandered it. Imagine how quickly you could re-pave a road if you know nobody is driving on it and you can close it as long as you need to.