In his pioneering work, Psycho-Cybernetics, plastic surgeon Maxwell Maltz showed that we usually behave in accordance with the image we have of ourselves.  Over time he noticed that the most important factor in a person's happiness was not what they actually looked like, but their self-image.  For example, if you view yourself as an unhappy person, your brain will make sure you experience life in accordance with that.

Maltz showed that self-image can be re-sculpted, little by little.  By visualizing yourself being happy for a few minutes each day (literally watching a movie of yourself in your mind), you can improve your levels of life satisfaction significantly.