A hedonic treadmill is the tendency of a person to remain at a relatively stable level of happiness despite a change in fortune or the achievement of major goals. As a person makes more money, their expectations and desires rise in tandem. So the rise in income results in no permanent gain in happiness.
The hedonic treadmill theory explains the often-held observation that rich people are no happier than poor people, and that those with severe money problems are sometimes quite happy. The theory supports the argument that money does not buy happiness and that the pursuit of money as a way to reach this goal is futile. Good and bad fortunes may temporarily affect how happy a person is, but most people will end up back at their normal level of happiness.
Credit: Investopedia