Hedonic treadmill

Hedonic treadmill is a phrase used to explain how people's moods tend to bounce back to a baseline level after going through an event that makes them feel happy or sad. Like someone might give you a gift, and you feel happy for a while, but then you'll eventually go back to feeling the same way you did before. It's like you're on a treadmill--you can make progress for a little while, but eventually you'll just end up back where you started.