Stress is not, of itself, a bad thing. But stress can come from many different sources and sometimes one stress can feed another.

We might be facing a deadline at work and that interupts our sleep pattern, which means we wake up the following morning not as rested as we usually are, which makes it harder for us to concentrate at work which makes it harder to meet the deadline which makes it harder for us to sleep at night which...  Well, you get the idea.

And the increasing pressure of the stress spiral acts like a lens to focus and intensify stress. Little issues become magnififed until they quickly reach a point of increasing the stress spiral until it becomes oppressive and difficult to focus on the difference between what is internal and external and it appears that everything is external and beyond our control.