Hedonic adaptation
Etymology
From "hedonic" (Greek "hēdonē," pleasure) and "adaptation" (Latin "adaptare," to fit to). The phrase, from 20th-century psychology, applies the biological idea of sensory adaptation — how a smell fades or eyes adjust to the dark — to emotional life.
noun phrase · /hih-DON-ik ad-ap-TAY-shun/
The tendency to return to a stable baseline of happiness after positive or negative life changes.
What it names
Why neither the raise nor the injury changes your mood for as long as you expected — the mind's stubborn drift back to normal.
Example
Lottery winners show hedonic adaptation: within a year or two, their reported happiness settles close to where it started.
Related idea
Hedonic adaptation is the underlying psychological mechanism; the hedonic treadmill is the vivid metaphor for its consequence — running after more and more pleasure only to stay in the same emotional place.