why would the author include a fable before the story even begins ?

The prologue acts as a fable to set up the story. It acts as a parable or a metaphor for the story you are about to read.This prologue introduces several elements which will come into play later in the narrative. First, the reader can see the attribution of human traits to inanimate objects - in this case, the lake that cries. This both sets the magical tone of the narrative and highlights the interrelation of all things, animate and inanimate, that forms one of the main themes of the book. Second, the story of the Narcissus highlights the question of selfishness, a question which is at the center of The Alchemist. Is it possible to pursue one's own personal desires while living a good life?