At the end of the story of The Alchemist, by Paulo Coelho, I believe that the alchemist leaves Santiago alone to complete his journey because he has done all that he can for the boy. Santiago has learned the Language of the World. He has already learned how to believe in his Personal Legend. He has also learned that if he wants something bad enough and works with the Soul of the World, he can realize his Personal Legend. The alchemist has nothing more he can share with the boy, nothing else to teach him. Santiago has learned, too, that he has found his true happiness and treasure in Fatima.
The alchemist taught the boy to look for God in all things.
Remember what I told you: the world is only the visible aspect of God. And that what alchemy does is to bring spiritual perfection into contact with the material plane.
I also believe that the alchemist knows that to realize his Personal Legend, Santiago must take those last steps himself: it cannot be done for him. In life, one must search for his dreams: no one can do it for him. So the alchemist leaves, though not without providing for the boy's well-being should he meet with trouble—which he does. Santiago comes to understand the secret to finding his treasure, he sees the Pyramids, and returns home to where it all began, all the "richer" for the journey.