Hedonic adaptation
The well-documented tendency of human happiness to return to a roughly stable baseline regardless of major positive or negative life events. Lottery winners and recent paraplegics, studied a year on, report happiness levels closer to their pre-event baseline than to the dramatic spike or crash one would naively predict. The phenomenon is sometimes called the "hedonic treadmill" because chasing further happiness through external changes tends, in the long run, to leave one running in place. Persistent irritants (a noisy commute, a bad marriage) are typically worse for long-run wellbeing than large but one-off shocks, because the mind never quite acclimatises to them.