Hedonic treadmill
Etymology
"Hedonic" is from Greek hedone, "pleasure." A treadmill was originally a grim prison device — a wheel prisoners trod to power a mill, walking endlessly without going anywhere. Psychologists joined the two in 1971: you keep striving for more pleasure but stay in the same emotional spot.
noun · /hih-DON-ik TRED-mil/
The tendency to return to a stable baseline of happiness after good or bad events, no matter how much changes.
What it names
Why the raise, the new phone, or the bigger apartment stops feeling special so quickly — the baseline resets and you’re walking in place.
Example
He upgraded apartments three times in five years and the hedonic treadmill erased the thrill each time within a month.
Related idea
Hedonic adaptation is the underlying mechanism; the treadmill is the memorable image for it.