So much of her lifestyle contributes a lot to the happiness that emanates from her.
1. She stays out of the public eye as much as possible - I never hear bad press, I never see her drunk or getting angry at paparazzi, and she always looks very put together. There have been a few paparazzi shots from the airport but since she isn't throwing a fit or looking like she's suffering from jet lag, Zooey probably doesn't get bothered very often.
2. She is a vegan - I'll bet there is something to be gained by ridding your body of harmful chemicals in processed foods. That's what keeps her glowing.
3. She's a musician -

Maybe I'm biased but I know that the first thing I want to do when I'm stressed is sit at the piano and play songs I know by heart and put my soul and voice into it. Not only does she perform but she writes her own stuff. Multi-talented.
4. She's married to one of the most brilliant musicians of our time - Ben Gibbard made HISTORY when he released
Give Up with The Postal Service. Our children will be listening to that CD when we are grey, chalky, falling apart.

Of course she's happy! Ben is a genius.
5. She's an actress - 'nuff said. B)