Santiago is a character who is focused on his own plans and the path he has to walk to fulfill them; he doesn't always take notice of what's going on around him because he is so laser-focused on accomplishing what he's set out to do.
At one point, the seer says:
The secret is here in the present. If you pay attention to the present, you can improve upon it. And, if you improve on the present, what comes later will also be better. Forget about the future, and live each day according to the teachings, confident that God loves his children. Each day, in itself, brings with it an eternity.
This is a lesson that Santiago needs to learn. He's always focused on what's at the end of the path instead of the present that he will always live in. That's a large point of the journey he takes. His dedication and focus are admirable but there are times when they put blinders on him.
When he arrives back at the church at the end of the book and finally finds his treasure, he wonders why the Alchemist couldn't have just told him where to find it in the first place. The voice answers and says, "If I had told you, you wouldn't have seen the Pyramids. They're beautiful, aren't they?" This is a large part of the lesson for Santiago: sometimes you have to pay attention to the things around you—the world and the people around you—and not just the things in front of you -- your goals.
Another example of this tendency is how Santiago isn't always focused on what is happening at the moment in his life. Instead, he focuses on what he wants. Paulo Coelho writes:
"Working," the boy answered dryly, making it look as if he wanted to concentrate on his reading.
Actually, he was thinking about shearing his sheep in front of the merchant's daughter, so that she could see that he was someone who was capable of doing difficult things.
He lives in his goals and in the future—not in the moment. This is why he pays so much attention to the things in front of him that he misses the things happening around him.