Stress is caused by two things: some kind of pressure and the choice to resist that pressure.
For example, if a co-worker tells you "You must finish this report in 20 minutes" you have two basic choices (and many choices in between). One choice is to accept the pressure and place yourself under stress by telling yourself "I must finish this report in 20 minutes". The other choice it to ignore the pressure and tell yourself "I will finish when it's done."
Of course, if this is a one-time request from your co-worker, you will be inclined to help out, accept the stress, and do the task faster. However, if your co-worker comes by every 30 minutes with such a request, placing yourself under stress every time will only lower your performance and put your health at risk.
We often cannot directly influence the pressure, but we can easily influence our reaction to that pressure. In other words, stress is primarily caused by our choice to accept outside pressure as our own.