From September 2018 to Christmas Eve, it seemed like the market was struggling to find a reason to buy Amazon (NASDAQ:AMZN). During that stretch, Amazon stock sunk from ~$2,000 to ~$1,300, and all those positive characteristics that made Amazon a trillion dollar company one day were quickly forgotten.

Since then, the market has found a lot of reasons to buy Amazon stock.

After bottoming on Christmas Eve, Amazon stock has since recovered in a big way, rising nearly 30% in about a month to over $1,700. The recovery in Amazon stock can be attributed to multiple things. Macroeconomic trends are improving. Political headwinds are gradually turning into tailwinds. Corporate earnings are better than expected. Amazon’s growth narrative continues to broaden and strengthen.

Overall, the reasons to buy Amazon stock are simply adding up. That’s why Amazon stock is flying high again. But, the best reason to buy Amazon stock is actually 101 million little reasons: Amazon Prime members.

Amazon Dominates E-Retail

Consumer Intelligence Research Partners data shows that Amazon Prime now has 101 million U.S. members.

That’s a big number. There are only about 117 million internet households in the U.S. Essentially, then, almost every internet household in the U.S. has an Amazon Prime membership. Granted, some of those 101 million memberships are college students — who wouldn’t count as households. There are also potentially multiple accounts per household due to various reasons.

But, the broad implication is consistent: Amazon Prime dominates the U.S. e-retail ecosystem, and this dominance is unprecedented for any industry. Netflix (NASDAQ:NFLX) has less than 60 million paid subscribers, while Spotify (NYSE:SPOT) has under 30 million paying subs. And Amazon of course makes money from all users, not just those who are paid subscribers.

This is a big positive for the entire Amazon ecosystem. The more Prime members, the more e-retail revenues Amazon generates. According to Consumer Intelligence data, Amazon Prime members spend more than twice on Amazon.com than their non-member peers ($1,400 per year, versus $600 per year for non-Prime members).

Thus, Prime growth is at the heart of Amazon’s e-retail growth. As the Prime base grows, Amazon’s dominance across e-retail will grow by two-fold due to wider reach and higher per capita wallet share.

Prime Is The Fuel For Everything Else, Too

Prime growth fueling big growth in the e-retail business is just the tip of iceberg. More Prime members also means more offline retail revenue, more digital advertising dollars, more logistics adopters and more potential pharma product buyers.

The rationale here isn’t hard to understand. The more Amazon Prime members, the more customers there are that are eligible for discounts at Amazon’s offline locations, like Whole Foods, Amazon Go and Amazon Bookstores. Also, on the advertising side, Amazon Prime members are essentially the same as active users, so the higher that number, the more advertising revenues will flow through the platform.

Meanwhile, Amazon Logistics (which should be coming soon) will inherently be tied to Amazon’s e-retail business. Thus, the more Prime members there are shopping on Amazon.com, the more potential adopters there will be for Amazon Logistics. Same is true with Amazon’s potential e-pharmacy business. Prime members already buy everything from Amazon.com. Why not buy pharmacy products from Amazon, too?

Overall, Prime growth is the fuel behind the entire Amazon ecosystem. As such, Prime growth is critical to the long-term success of Amazon stock.

Bottom Line on AMZN Stock

Amazon has its fingers in everything that matters, from e-retail to digital advertising to logistics. All of those businesses are fueled by Amazon Prime. Thus, as goes Amazon Prime, so goes Amazon stock.

Right now, Amazon Prime is huge and only growing. That means Amazon stock will keep growing, too.

As of this writing, Luke Lango was long AMZN, NFLX, and SPOT.