The stresses of life at times most of us have to suffer...
I suspect part of this increase in mood up to $75k but not beyond that point is simply due to the stresses in life of having to pay bills and living expenses without enough money to go around. Something that is obvious to everyone without enough money to go around!. People on low money have to worry endlessly about each new bill and how to pay just to live whereas once you earn enough not the have to worry about the bills, that is such a freeing experience, so then earning even more isn't going to make much more difference in terms of the every day stresses of life.
The problem is *most* employers want to keep *most* of their employees well below this $75k because (A) they want all that money for themselves, and (B) keeping staff pay low gives a boss a way to exert more pressure on employees to make them work harder for their living (although point (B) is something that I find usually only more experienced workers come to realise, after they have been through a few companies, as they start to see the patterns repeating in each company).
Often younger people say things like, "the richer people in society don't know just how hard it is", and while at times that's partly true, it however masks a far bigger issue that many richer people do understand how hard it is for most workers and they actually secretly want it that way. Business thrives on pressuring people to do what the people with the money want. Ultimately money is a form of power. Its power over people who need money to live and the more desperate they are for money the more power money gives the people with money. That's simply life, but it is very stressful to be forced to live like this, hence the results of this study make sense. But these results are obvious to anyone who has to live like this!