The purpose of stress is to quickly adapt the body to survive a fight. A set of changes happen to the body.

For example, the blood becomes thicker in order to clot faster when you are bleeding. However, with chronic stress, this can cause thrombosis.

Under stress, the stomach stops the digestion so that the energy goes into running to flee the danger. With chronic stress, your stomach is messed up and you get all sort of indigestion.

And so on. Many examples of changes that have a simple explanation but are hazardous in chronic stress situations.

For reference, read the Why Zebras Don't Get Ulcers or anything by Robert Sapolsky.

Acute stress keeps you alive. It's a response to esp. novel and potentially harmful events. Its effects on cortisol/insulin -> diabetes however, seems to be contentious.