There is a great article in the New Scientist this month which looks at the effect of the mind on health.
THE POWER OF BELIEF
Belief can make a medicine have positive effects, even if it has no active ingredients. Most people are familiar with this 'placebo' effect. But recent studies suggest that this can happen even if the patient is TOLD that the medicine has no active ingredients. All that is necessary is that the patient believes that there can be a positive effect.
It is well known that optimism protects against stress - for example, by lowering cortisol levels. But some research has gone further, showing positive effects independent of mere stress-reduction. The body seems actively to get better at maintaining and repairing itself if you have a positive outlook. Additionally, as well as having a positive outlook on life, having a positive view of yourself seems to reduce levels of adrenaline and other fear hormones.
There is an increasing body of evidence showing that meditation protects against depression, anxiety and disease, and may even make you live longer. Structural changes in the brain can happen after only around 11 hours of training.
Although we don't know why hypnosis works as a therapy, it seems to operate through tapping into these same positive effects of belief. It can help with mood, eating issues and skin disorders, among other things.
If you are happy with your social life, and are with others you trust, evidence suggests you sleep better, stay younger, and respond better to many medicines. Evolutionary researchers have suggested that loneliness triggers chemical responses (e.g. cortisol) related to defence against injury. In contrast, social interaction may trigger chemical responses to protect against transmitted diseases (e.g. viruses). In this way, good friends can promote immunity!
DO WHAT YOU LOVE
Researchers disagree about the direct effect of religion, and spirituality. But some are convinced that living a life which you find meaningful, and doing what you love, can in themselves bring key health benefits.
In summary, it seems that a positive attitude, towards life, yourself, and others, can provide a significant health boost. More than that, I'd say, it's just a nice thing to do!
This is a very brief summary of an original article in the August 2011 edition of New Scientist ('Heal Thyself', p.32)