Sorry, no citation, but I remember reading of a study years ago that basically determined that no matter how much income a person has, they feel that they need

Sorry, no citation, but I remember reading of a study years ago that basically determined that no matter how much income a person has, they feel that they need about 10% more to be truly happy. This was consistent across a wide range of income. Honestly, I think it is simply normal human psychology to keep us striving. Complacency is not an evolutionary stable thing to do.