Interesting Ted Talk with Prof. Dan Gilbert

To summarise the video – happiness is basically internal. There is a theory called the adaptation principle, which suggests that whatever happens to us externally in our lives, we always return to a baseline level of happiness.

There was a study in 1978 that compared lottery winners to accident victims, and found little difference in their happiness levels after 6 months:
http://www.ncbi.nlm.nih.gov/pubmed/690806