The stress response is triggered when you believe yourself to be under threat. It is a survival mechanism.
When triggered the brain releases cortisol. This increases heart rate, heightens blood pressure, increases production of fatty cholesterol and suppresses the immune and digestive system.
This highly automated response enables you fight the threat or escape. Thousands of years ago it saved our lives. Today the physical effects of stress cause heart disease and cancer - amongst many other side-effects.
You use the automated thought process of cognitive appraisal to evaluate threat. Today, this process can trigger the release of cortisol routinely. A disliked boss or colleague; screaming children, even a passing motorist can be a trigger. Unpaid bills, relationship issues, a delayed train… you name it almost anything can lead to stress - with all the resultant issues.
None of these people or things causes stress - how you evaluate them pulls the trigger.