why must santiago go on his jorney before discovering the treasure

He must go on his journey first in order to find the spiritual force that connects one with the universe.

“…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."

This statement, which Melchizedek says to Santiago upon their first meeting, forms the foundation of the philosophy of The Alchemist. Essentially, Melchizedek says that dreams are not silly or selfish desires that should be ignored. Instead, they serve as the primary means by which people can get in touch with the mystical force that connects everything in the universe. He convinces Santiago that his nagging desire to visit the pyramids is actually a calling, and he sets Santiago on his journey of spiritual discovery. By associating seemingly selfish human desires with the soul of the universe, The Alchemist presents a form of spirituality that differs radically from traditional religions that espouse self-denial. Instead of practicing sympathy by identifying with and helping others, Santiago must focus on his own personal dreams.

This quote also introduces the concept of the soul of the universe, which characters refer to later in the novel as the Soul of the World. This entity becomes extremely important later in the book, as it is the spirit that Santiago must connect with in order to turn into the wind. The quote alludes to the idea that a person’s purpose in life centers on fulfilling one’s desires, a notion that also becomes important in the form of the Personal Legend. Although this quotation doesn’t mention these ideas by name, it lays the groundwork for Santiago’s and the reader’s later understanding of them.