Today I Was Thinking

Today I was thinking, why is most things all in our head?  Why do we let anxiety and depression get to us when nothing is hurting us expect ourselves with all these thoughts.  I know I overthink pretty much everything.  I get overwhelmed usually that means I'll have an anxiety attack.  Why are mental disorders existent?  Things happen for a certain reason.  There can be different types of impacts in your life that can lead to this.  What were you thinking?