From 06.16.2003 - Can money buy happiness? UC Berkeley researchers find surprising answers:

"Perhaps making a lot of money in your job can actually cause you to question why you are working at the particular job you have, even if you chose the job for intrinsic reasons," Malka said. "There's a substantial psychological literature showing that receiving monetary rewards for doing a fun task can make the task seem less enjoyable. This past research suggests that your sense of how fulfilling and personally rewarding you find a task is very fragile, and money can shake this delicate sense of enjoyment."

"Individuals have a fundamental psychological need to feel as though their actions are freely chosen," the authors wrote. "In other words, we all need to feel that we are not just doing the work for the money, and intrinsically motivated individuals need to feel this even more so," Chatman added.