I'm not the guy you're asking, but I'll weight in. I think happiness is much more related to self-esteem and self-efficacy than it is to how much money you have.
For this reason, I think it's common for people born at any station in life who are successful relative to that starting point, to be happier than people born at a higher station that are not successful or are motivated by the wrong things.
For this reason, I think it's common for people born at any station in life who are successful relative to that starting point, to be happier than people born at a higher station that are not successful or are motivated by the wrong things.