Hedonic Treadmill

The hedonic treadmill is the phenomenon that humans quickly return to a stable level of happiness or sadness even after major positive or negative events. It's been widely studied by psychologists and the effect has been seen across a wide variety of settings and with a wide gamut of events.

One explanation for the hedonic treadmill is that expectations rise in conjunction with circumstances. For example, if you earn more money, your expectations of how much money you should earn also increases. Another explanation is that it may be evolutionarily adapative for our minds to work this way. Let's say an animal experiences a negative event-- for example, a serious injury. After the immediate danger has passed, it's often the case that the best way forward is to return to normal-- continue seeking out food, shelter, mates, etc. And at the opposite end of the spectrum, say an animal discovers a large cache of food. There's still evolutionary pressure to seek out even more food, because seeking more food provides redundancy if the cache is ever exhausted.