Reasonings behind hedonic adaptation

... also referred to as hedonic treadmill, is defined as "the observed tendency of humans to quickly return to a relatively stable level of happiness despite major positive or negative events or life changes"  - Positive Psychology Program.

After a while, people become used to change...

We quickly return to a relatively stable level of happiness despite positive or negative external events. We pursue a promotion and believe it will make us happy. When we get it, we are temporarily happier, only to get back to our baseline levels the next week.

• People who win the lottery are likely to revert to their original levels of happiness after the novelty of the win has worn off.
• It is also true for those who are in major accidents. People generally tend to return to their pre-accident levels of happiness after a period.