From Wikipedia:

The hedonic treadmill, also known as hedonic adaptation, is the supposed tendency of humans to quickly return to a relatively stable level of happiness despite major positive or negative events or life changes. According to this theory, as a person makes more money, expectations and desires rise in tandem, which results in no permanent gain in happiness. Brickman and Campbell coined the term in their essay “Hedonic Relativism and Planning the Good Society” (1971). During the late 1990s, the concept was modified by Michael Eysenck, a British psychologist, to become the current “hedonic treadmill theory” which compares the pursuit of happiness to a person on a treadmill, who has to keep working just to stay in the same place.