2 points here: First, health and happiness (and success, too, btw) are self-reinforcing -- if you have more of one, you tend to have more of another. And there are many studies to support this.
Second, since this is true, and since my friend's happiness affects mine, then my friend's health affects my happiness, which affects affects my health. This is a great reason for universal health care. Perhaps this is why there are so many first world countries that are happier than the US. -- they have universal health care, and so they are healthier, so they' re happier.
Social Medicine - Opinionator Blog - NYTimes.com