You have to consider that things people are expected to buy become more expensive through time. So if their income gets a litle better they are still in some way becoming poorer.
For instance kids expect to have ipads and iphones, videogames, new clothes, etc., a lot more than before. We can´t expect parents to not spend with these things more than 20 years before, that is a given.
Also if girls want to spend more on makeup and hair, if they are all expected to look “better” (i.e , more beauty spending), that also becomes a factor (even to males, who many times “have” to pay for those things).
People dont have to spend more on kids, but if they don’t their kids will feel unhappy. With wives and girlfriends that can be even more cruel since not spending the ammount they expect can lead to losing them, making the competition even more cruel. Even if it doesnt lead to losing them, you can’t risk that so you have to spend more. Those are just two examples.
The relative gains of people become really relevant when we take that into consideration. What is money for, apart from making more money? Mostly for women and children, wives and kids. The ammount they require always depends on the ammount other kids and other women are receiving.
That silent competition, specially cruel among poorer people and lower middle class folk, creates an importance of relative profit that rich people always seem to ignore — because their women make more money and have more “pride”, while their kids can be controlled better(they can expect more from cooperating with parents, who have also more time and knowledge on how to control them).