Why do people overestimate future happiness when they imagine acquiring something like a house or winning the lottery? When a person says he would be happy to win the lottery he is usually wrong?
Why are people bad at judging their happiness in the future? A person who knows that he will get a fancy house in the future thinks he will be happy. But when he ends up living in it he is not as happy as the thought he would be.
Like many people, I do this a lot, the whole thought process "If I won the lotto...".  Following on from the term that AsheeshR linked in his comment, there has been several studies on the phenomenon of the Hedonic Treadmill.

One particular study is "Getting off the hedonic treadmill, one step at a
time: The impact of regular religious practice
and exercise on well-being" (Mochon et al. 2008), where they assert that a sudden large event, such as winning the lotto gives an initial burst of happiness, but we quickly adapt, from the article:


  Many studies have shown that few events in life have a lasting impact on subjective well-being
  because of people’s tendency to adapt quickly; worse, those events that do have a lasting impact tend
  to be negative.


Whereas, regular small-scale events, using the examples provided in the article: exercise, religious attendance, and I'll add seeing a good movie, meeting an old friend etc may do so


  by providing
  small but frequent boosts: if people engage in such behaviors with sufficient frequency, they may
  cumulatively experience enough boosts to attain higher well-being.


The authors of the article suggest that major events, such as winning the lotto do not promote a lasting sense of well being as do frequent smaller scale events.

An earlier study that found similar results was "Lottery winners and accident victims: is happiness relative?" (Brickman et al. 1978).