Steinbeck was a great scholar of human nature. His allegorical tale of Kino and the pearl demonstrates a fundamental part of human nature: people always want what they can’t have.
Everyone wants something from Kino after he finds the pearl. It’s both extremely valuable and worthless. As a large pearl, it should be valuable. However it’s not worth anything because he can’t sell it. Everyone just wants to take advantage of him, and find a way to get his share of the pearl. So even though Kino has a valuable pearl, he doesn’t have the money to save his baby. That is why he throws the pearl back into the ocean.
However, as the second half of the quote addresses, there is an advantage to always wanting more than you have. If humans never dreamed, we would never accomplish anything. It is our dreams that make us who we are. Even though Kino cannot sell the pearl, the dreams of what he could do with it mean something. They may be all he has.