When I first heard that, it just blew my mind. I think that's the philosophy that keeps Amazon moving upwards. At so many points in Amazon's history you can imagine the story ending, but 5 years later it turns out to have been just the beginning, just the introduction.
Successfully starting a bookstore wasn't the end, it was day1 for a music store. The music store wasn't the end, it was day1 for a retail store. Retail was day1 for digital downloads. Downloads were day1 for Kindle. Prime, AWS, India, Android, Same day delivery, groceries...
When you tell yourself the story of your business, you probably start with the idea, go through the struggle, and end with your current success. Bezos puts all of that in the first paragraph, and then tries to imagine the rest of the book.
"It's always day one."