AND yet SSRIs are effective in treating depression and anxiety disorders.
There are degrees of sadness to depression and stress to anxiety. Some are short term and “life events” (ie death in family, moving, divorce, etc) and some are chronic or physical (medicines for disease conditions or the diseases themselves).
FOR example: Most of the serotonin found in your body is in your gut (intestines). About 90% of serotonin is found in the cells lining your gastrointestinal tract. It’s released into your blood circulation and absorbed by platelets. Only about 10% is produced in your brain.
How do you think Crohns may effect this hormone production? Jogging sure in fuck doesn’t create it 
Perhaps narcissists can get depressed? So CAN empathetic humans who don’t wallow in self-pity and absorption.
Edited to add: layman’s understanding (food, exercise play apart)