Why is The Hero's Journey also considered as The Monomyth ?

The reefs, by marcella morristhe theme of this    story is to never give up on what you like to do, work hard and to always have good supporting friends. in the story sop hie trip she took with her aunt to the coral reefs when they get there a snorkeling  guide told them that all the  coral reefs will not last very long. sophie cared about the coral reefs and the habitat that lives  there. so she wanted to make a change, she worked as hard as can could to find way  to save the coral reefs. she was scared to show what she did but  she friend she right there to give she support. this evidence made me coming up with my theme for this story.