Hedonic treadmill, also known as hedonic adaptation, describes the tendency for human happiness levels to be relatively unaffected long term by major life changes, either positive or negative. Essentially an individual's happiness is like a treadmill - although things keep changing, for better or worse, they essentially stay in the same place. A concept initially developed centuries ago it is a major focus in the field of positive psychology.
For example, a person wins the lottery which drastically increases their happiness levels for awhile. But shortly afterwards their stress beings to increase: their family and 'friends' begin to come out of the woodwork asking for money, they realize they have to spend so much time and effort managing their money, people attempt to take advantage of them, they begin to wonder if people like them for themselves or only because they are rich, etc. Shortly after their win their general happiness levels return to what they were before they won the lottery as the negative aspects of this situation temper the positive aspects. Hedonic adaptation has occurred - they have returned to their hedonic set point (sometimes called the happiness set point) which is essentially their baseline for happiness.