The surprising reason I re-upped my Amazon Prime subscription

My subscription to Amazon Prime - the company's $99-a-year membership service which boasts free, two-day shipping on millions of items - ran out last week.

Once I realized that the time had come, I contemplated whether or not I actually needed to re-up the service. Afterall, I don't buy that much from the site.

During the 12 months I paid for Prime, I purchased 31 items, meaning I likely got my money's worth on shipping alone. However, when I looked over my order history, I found a bunch of random items that I either really didn't need or could have just as easily bought in the real world.

If I didn't dish out $99 to re-join Prime, I thought, I'd likely cut out a bunch of impulse buys, like that gaudy gold lamé jacket or my endless supply of candles.

Plus, my subscription also ran out on the exact same day as Amazon-competitor Jet.com launched, and for a while I mulled over joining its shopping club instead, which promises prices 10-to-15% cheaper than Amazon's.

But then I got a message that convinced me to succumb to Prime's consumerist pull:

That's the part that I didn't mention before: Besides two-day free shipping, Amazon Prime also comes with access to thousands of free TV shows, movies, music, and books, as well as unlimited cloud storage for photos.

Personally, I never use the Kindle Lending Library and I haven't gotten addicted to any Amazon TV shows, but I'm an unabashed Prime Music devotee.

Even though Spotify has a much bigger selection - a catalog of 30 million tracks versus Amazon's comparatively paltry 1 million+ - I've been happily using Prime Music for months and had downloaded about 1,000 songs for offline listening.

So, when the aforementioned error message kept coming up when I went put on some tunes during my commute or while cleaning, I felt crushed.

I've resisted signing up for Spotify Premium, Apple Music, or any other paid streaming service because the free Spotify experience is pretty great on desktop and I don't listen to that much music on mobile (I'm more of a podcast person).

I fell in love with Amazon Prime Music because it was "free" - it just came with the shipping service I was signed up for anyway. And, ultimately, my desire to get my "free" music routine back was the tipping point that convinced me to shell out $99 for Prime afterall.

In other words, I'd fallen into Amazon's trap.

I call my decision to renew Prime because of its music offering a "surprise" because several people I mentioned it to seem shocked at my rationale, but it's exactly what Amazon has been betting would happen.

Over the years, the company has been slowly building up the additional services that it offers with Prime, even though the traditional retail component is its bread-and-butter. It loses a lot of money on the TV and music streaming services, but it has built them up to make Prime more attractive and addicting. Amazon knows I'm more likely to make more impulse buys if I pay for Prime: Members may spend more than double on the site per year than non-members do, according to an analysis from Consumer Intelligence Research Partners in January.

After Amazon spent more than $100 million on original video content last year, CEO Jeff Bezos put it bluntly when talking about Prime's award-winning show "Transparent": Amazon's the first company to use a Golden Globe to sell toilet paper.

On the one hand, I feel a little duped that I'm paying more money to probably end up spending more money, but at least I'll be doing it do a sweet soundtrack.

Disclosure: Jeff Bezos is an investor in Business Insider through hispersonal investment company Bezos Expeditions.