Humans tend to return to baseline levels of happiness no matter their circumstance.
The Hedonic Treadmill is a phenomenon In which humans tend to return to their own individual baseline levels of happiness, regardless of what their life circumstances are. There is no such thing as “happily ever after”. There is no such thing has “permanent pain”. Humans adapt to whatever their circumstances are, this becomes their new baseline, and their day-to-day happiness is not really affected.
The standard level of happiness each individual tends to return to is referred to as their “Hedonic Set Point”.
The term “treadmill” is used to refer to the fact that you are always running forward toward more happiness, but you will never go anywhere. Buddhist philosophy would agree that this is a futile and ultimately harmful effort.
Implications
If you win the lottery today, or if you lose a finger in a fight with a lawn mower, after ~1 year You’ll be more or less equally happy.
If you buy something, or suddenly make more money, you’ll maybe be happier for a bit, but quickly return to your baseline.
Exceptions
This, like most things, is not without exceptions. Major disabilities and other life events (marriage, divorce, parenthood, imprisonments) can durably adjust the set point.