I think a large part of what it is, is the want for more. Once you reach a certain point of wealth, you realize just how possible it is to get just a bit wealthier, but doing so means putting off your kids for a couple years. And you get there, you now have a BMW... but now you want a nicer house, and everyone tells you about being living within your means, and the way everything looks right now, if you play your hands right, you can get that new house in just a couple years, if you put off your kid again... and repeat.
I have no actual proof of this, but it's just a generalization of some human behavior I notice. Namely, you get much more interested into a "game" once you invested a certain amount of effort/have a certain position. For example, someone of average fitness doesn't really care about shaving 20 seconds off their 5k, or that extra 5 pounds of benchpress. But the person who is waay more fit will invest incredible amounts of effort/time/focus into those goals.
I have no actual proof of this, but it's just a generalization of some human behavior I notice. Namely, you get much more interested into a "game" once you invested a certain amount of effort/have a certain position. For example, someone of average fitness doesn't really care about shaving 20 seconds off their 5k, or that extra 5 pounds of benchpress. But the person who is waay more fit will invest incredible amounts of effort/time/focus into those goals.