"If you change your thoughts,
You change your life,
And you change the world."
Imagine you are a zebra and a lion has just leapt out and ripped your stomach open. You've managed to get away and now you have to spend the next hour evading the lion as it continues to stalk you. Or perhaps imagine you are the lion, half starved, and you had better be able to sprint across the savanna at top speed and grab something to eat or you won't survive.
As humans we can experience wildly strong emotions from mere thoughts, provoking our body into an accompanying uproar. Two people can sit facing each other doing doing nothing more physically strenuous than moving little pieces of wood now and then, yet this can be an emotionally taxing event such as the chess grand masters. During this tournament it can place metabolic demands on their bodies that begin to approach those of athletes during the peak of a competitive event.
So if you are a zebra running for your life, or a lion sprinting for your meal, your body's physiological response mechanisms are superbly adapted for dealing with such short term physical emergencies. For the vast majority of beasts on this planet, stress is about short term crisis, after which its either over with or your over with. When we sit around wording about stressful things, we turn on the same physiological responses - but they are potentially a disaster when provoked chronically. A large body of evidence suggests that stress-related disease emerges, predominantly, out of the fact that we so often activate a physiological system that has evolved for responding to acute physical emergencies, but we turn it on for months on end, worrying about mortgages, relationships, and promotions.
Let's think of the idea of homeostasis for a minute. Homeostasis is the ability or tendency of an organism or cell to maintain internal equilibrium by adjusting its physiological processes. Such maintaining ideal levels if oxygen, acidity, temperature and so on. And a stressor is anything in the outside world that knocks you out of homeostatic balance. A stress response is what your body does to reestablish homeostasis.
Now of course I personally believe that our lives are completely different from a zebra in one way but of course I know that these are some great concepts and that it is completely unnecessary to worry about as many things as we do. Treating everything as an emergency. Living in a state of emergency creates stress in our lives and when you live in a state of constant stress it causes physical and mental illness and unhappiness. Now it seems almost all the stress created is within our own mind. How we perceive things. The expectations we have for ourselves and one another. Thinking that you know how things should be will leave you in an almost constant state of disappointment vs. just accepting them as they are.
Most of this information came from the book: Why Zebras Don't Get Ulcers written by Robert M. Sapolsky. A fantastic book about how the the body reacts to stress.