Too much stress can is linked to various heart, digestive, and other diseases. Let me backtrack. These are not undesireable effects if you meet a bear in the forest and have to act quickly in order to save yourself from danger. The heart rate speeds up in order to quickly provide the extra oxygen and nutrients your body will need. Blood pressure rises as vessels to unneeded parts of the body constrict.
Respiration speeds up to provide extra oxygen.
Cholesterol and triglycerides provide the building blocks for many metabolic functions. The immune system shuts down because it's not needed at the moment. The brain goes on high alert. The stress response has saved your life. The problem is that sometimes a stress demand doesn't go away.