what goes on in your head affects your health, immune system etc. Want to read up on psychoimmunology:

“In his work with animals Cannon observed that any change of emotional state in the animal, such as anxiety, distress, or rage, was accompanied by total cessation of movements of the stomach.” (wikipedia: immunology)

your mind and happiness: the way you interpret things!! what’s in your head has great influence over your experience. we can just change our mind, not the entire world.
avoid excessive focus on oneself.
generosity and peace, looking out at bigger world.
be part of something.

http://www.youtube.com/watch?v=L_30JzRGDHI&feature=channel

I think i might get sick a lot less if i were happier/healthier mentally. Psychoneuroimmunology studies showed that cetain stimulus can supress your immune system.
http://en.wikipedia.org/wiki/Psychoneuroimmunology