Hassan's the man at the microphone in this book, telling us his own life story through his own eyes and experiences. That Hassan is running the show is clear from the very first sentence. Check it out:
I, Hassan Haji, was born, the second of six children, above my grandfather's restaurant on the Napean Sea Road […]. (1.1)
You see that I that kicks the whole thing off? Hassan is talking about himself in this one, start to finish. It's a pretty formal opening—it's got a testimonial-ish vibe—which sets us up to take Hassan's story seriously as he shares it. This is fitting since, though not driven by ego, Hassan definitely takes himself seriously. Otherwise he wouldn't wind up so darn successful.
A key component to first person narration is that it gives Hassan—as both the main character and the person recounting the tale—the ability to customize how his story is told. So while someone who isn't Hassan might stick to the facts, Hassan busts out food references left and right, and in doing so, we get a better sense of who he is and how he goes through the world. A different narrator might not notice all of the food and connections to food that Hassan has, but he doesn't seem to miss a morsel.
Reliability is always a risk when you have a first person narrator, but Hassan is pretty darn observant and detail-oriented, in addition to being readily interested in other people (more on this over in the "Characters" section). This means that not only does he call things like he sees 'em, but he spends time sharing the spotlight with different characters. This helps us get a better sense of the people Hassan comes in contact with than if, say, he opted to always stay in center stage himself… even if what he tells us isn't always a proper firsthand account. As he clarifies:
The story I tell is God's truth, even if I did not witness every event firsthand; the fact is that many of the details of my own story were revealed to me only years after the fact, when Mallory and the others at long last told me their version of events. (5.1)