Watson, half of the team that discovered the double-helix structure of DNA, interviewed in a recent issue of the New York Times magazine:

…I have an odd theory on happiness, and it bothers people. My general theory is that happiness is a reward for an animal doing what it should be doing. So if a horse runs, it feels happy. Or if you are too thin you can’t be happy, because evolution wants you to be tense and anxious, trying to wake up in the morning looking for food. So I was just saying that happiness comes only when you are doing things that are good for you….

…The molecule that you make when you are getting sunburned or when you eat a lot of food is part of the same molecule that contains an endorphine or an opiate. No one has ever had a hypothesis about why the two are together. So I came up with one.

And your hypothesis is that sun and food make you happy?

Yes, and running and exercise.

More on his theories, link courtesy of Bill.