The alchemist tells Santiago a story about the Roman Emperor Tiberius. One night an angel came to Tiberius and told him that the words of one of his sons will live on for generations. One of Tiberius's sons is a poet so naturally he thinks that the angel is referring to him, rather than his other son, a soldier.
But after Tiberius dies and his spirit meets up with the angel again he is surprised to discover that it was the words of his soldier son, a Roman centurion whose servant was healed by Christ, that have lived on:
The centurion answered and said, Lord, I am not worthy that thou shouldest come under my roof: but speak the word only, and my servant shall be healed.(Matthew 8:8 KJV)
The moral of this fable explains why Santiago must encounter the dangers of a tribal war before arriving at the pyramids. Like the Roman centurion, Santiago is living out his Personal Legend and in doing so he is serving the Soul of The World. What matters in life is not getting what you expect but instead following your desires, irrespective of risk. Doing so will lead to unexpected outcomes, and you will be serving a higher purpose.
But this can only be done by the individual. Everyone has their own Personal Legend and must achieve their dreams by themselves. Yet this is not mere selfishness; the alchemist leaves Santiago to live out his dreams safe in the knowledge that in doing so, Santiago will light the way for others to fulfill their own desires.