Stress causes the release of hormone called cortisol. Cortisol can depress the immune system and also raise blood sugar. The body can become addicted to it and try to keep it elevated. There is evidence that neurones in the brain shrink in response to high levels of stress hormones, causing permanent changes in the brain.
Numerous studies attest that stressful life events such as death of a spouse or parent, marriage, divorce, desertion, loss of employment, birth of a handicapped child, etc, often precipitate physical as well as emotional illnesses. For example, the death rate of widows and widowers during the first year of bereavement has been shown to be 10 times that of other persons of the same age; divorced persons, in the year following their divorce, have an illness rate approximately 12 times that of married persons of similar age and situation. Young married women have exceptionally high referral rates to medical services, particularly for depressive and neurotic conditions, which raises many questions regarding the demands made on the housewife with a young family in a mobile, urban society. More in keeping with expectations are the high referral rates of the elderly to the psychogeriatric services because of disorders directly attributable to recent social problems, often of a kind that, with a little support from services, relatives, or neighbours, could have been remedied. Moreover, there are grounds for supposing that psychosocial stresses precipitate all kinds of illness, not simply mental ones. It has also, however, been very clearly demonstrated that people with depression and with schizophrenia, in the months immediately preceding the onset of their illness, experienced more stressful events than did a control group followed up for the same period.
Remember, even if you win the rat race, you're still a rat.
Stress is often good for people. It keeps them from degenerating into mindless blobs. Too little stress, namely prolonged boredom, stagnation and lack of stimulation, is more unhealthy than too much.