Why did paolo coelho write the book of the alchemist?

Central to the novel is the concept of a Personal Legend. Santiago first learns of one's Personal Legend from The King of Salem, who tells him "It's what you have always wanted to accomplish. Everyone, when they are young, knows what their Personal Legend is."[8] He expounds on this saying "...there is one great truth on this planet: whoever you are, or whatever it is that you do, when you really want something, it's because that desire originated in the soul of the universe. It's your mission on earth."[8] The King also tells Santiago of the importance of following the omens on the journey to realizing one's personal legend. Coelho follows this advice himself; he will only start writing a book after finding a white feather.[9]

The Alchemist is basically a reflection of Coelho's life.