Why does Coelho open with the modified myth of Narcissus? How does the new version differ from the original one? How does it change the myth’s meaning? What might the author be suggesting about how we perceive ourselves and the world?

Coelho's opening with a revised version of the Narcissus myth invites us, as the story begins, to be open to seeing the world from a new perspective.
In the original myth, Narcissus is beautiful and proud. One day, he sees himself reflected in a pool of water. He doesn't realize he is seeing an image of himself and falls in love with it, believing it to be somebody else. Eventually, he dies of longing and is turned into the flower called narcissus.
In Coelho's retelling, this judgmental tale of self love punished is turned around. We see Narcissus from the point of view of the pool of water. The pool mourns Narcissus's death because his beauty, reflected in the pond, was a gift to it. The message is that we all have gifts we bestow without even knowing it. In a book that dwells on the positive—discovering and embracing your own Personal Legend—this positive retelling of an old myth sets a tone that asks us to rethink what we think we know and to be open to possibility and the life lessons that surround us.

The original Greek myth about Narcissus tells the story of a beautiful young man who becomes obsessed with his own reflection. In Edith Hamilton's Mythology: Timeless Tales of Gods and Heroes, a chapter about the origin of flower names explains how Nemesis, a goddess of anger, curses Narcissus after he rejects the love of a nymph named Echo. He is cursed to only love his own reflection, and he dies beside the water bank, staring at his own reflection. After his death, the nymphs name the flower that grows on the bank after Narcissus.
The story ends here in classic mythology, but Coelho adds to the story, giving the body of water that Narcissus stares into a personality and agency in the ending. In Coelho's version, the lake is sad about Narcissus dying, and it transforms into a saltwater lake, a lake of tears. When asked why he is sad, he says it is because of Narcissus's death, because by the reflection in Narcissus's eyes, the lake could contemplate its own beauty.
The original myth contains a critical theme about vanity and selfishness, revealing Narcissus to be punishable and cruel for his rejection of Echo. Coelho's myth doesn't mention that Narcissus was cursed at all, only that he loved himself and that the lake loved itself as well because of what it could see of itself in Narcissus. This shifts the message away from criticism and more toward a theme of contemplation. It begs the reader to consider how we learn about ourselves through the eyes of others, or perhaps it asks us to consider our dependence on outside sources to understand ourselves.
As for Coelho's intention for including this preface, considering authorial intent can always help the reader to make an educated guess. Since The Alchemist contains themes of self-discovery and dependence upon the universe to find personal fulfillment, it is reasonable to conclude that Coelho intended to start readers' minds upon the idea that everything is connected in the same way that Narcissus needed the lake and the lake needed Narcissus. Santiago, in his journey toward his treasure, needs many people to find his way and learn his mission. This altered myth suggests that the people Santiago meets along his path need him in much the same way that he needs them.