People are never satisfied with what they have, they either yearn for the happiness one can get in life or the wealth one could possess. Which is why the wealthy and famous are always whining and as a response, the less wealthy are confused as to why the people who basically have everything they need, are still not content with their lives.
The answer is never really simple, but it could be argued that the more successful and richer you are, the more greedy and under appreciating you will be. I say this because being successful raises your standards and nothing will ever be good enough when you believe that you can do better.
This article that I read does address some facts about why the wealthy are more unsatisfied compared to the less wealthy, although we can all agree that numerical figures do not account for something completely and every individual is different, but the figures do give you an interesting insight into what would normally be seen.