Earlier quoted context omitted.
Depends on where you are. After all, living in a rich environment might get you better public services. And it's not like you exchange salaries with everyone you meet.
It's a thought experiment. Assume that both neighbourhoods provide the same services.
Challenging yourself to earn more, and actually seeing the numbers rise, is where the joy comes from, at least in my experience. If you are challenging yourself and your income isn't rising (or falling, even) it is pretty disheartening. If you aren't challenging yourself, I expect life becomes pretty boring. Either way, I can see why happiness is linked to income deltas, even once the basic means are covered and presumably people with higher incomes have seen more changes.