LiveScience (one of my favorite websites) has an article up explaining why monks are so darn happy. According to them, and research that has been done in recent years, monks are happy because they spend their days meditation and learning to have compassion for all people.
Meditation really does make you happier, along with making you more relaxed and focused. I try to meditate every day (although I've been neglectful the past week), and I think I'm a much better person overall then I was before I took up meditation.
Why Monks Are So Darn Happy
Beginner's Guide to Meditation