Our systems evolved in order to allow us to respond to immediate danger but with the further development of our brains we have acquired the ability to fire our Stress-Response (Flight of Flight) just by thinking about things that might happen to us in the future.
This is a very good of a documentary by National Geographic on Stress.
If you get a chance you should read any books that you can get your hands on by Robert Sapolsky.
The book that I am currently reading, "Why Zebras Don't Get Ulcers".