While I'm a big believer in diversification and currently hold positions in more than 70 individual stocks, my portfolio is actually more concentrated than you might guess. In fact, my top three holdings -- Amazon.com (AMZN -1.43%), Intuitive Surgical (ISRG -0.26%), and The Priceline Group (BKNG -0.43%) -- currently comprise more than 21% of my portfolio's total value.

Why have I allowed these three companies to make up so much of my net worth? Below is an overview that explains why I'm so bullish.

Hands pushing poker chips all in

Image source: Getty Images.

The Priceline Group -- 6% of my portfolio

It took me quite some time to warm up to the idea of investing in The Priceline Group. After all, consumers have hundreds of online travel sites to choose from when they want to book a vacation. That's why I had a hard time figuring out what made this company's sites so different from all the others.

I finally learned the answer to that question in 2010, and I started to accumulate shares in the company that year. The Priceline Group's golden goose was a site called Booking.com, which the company had acquired in 2005. This site had a runaway lead in the European hotel market, having spent years to build out its network. That decision proved to be a stroke of genius since many European hotels are mom-and-pop operations, not franchises owned by the big boys.

By amassing the biggest collection of hotels, Booking.com earned a reputation among travelers as the go-to site if they wanted to maximize their selection. In turn, hotels learned that they needed to be on Booking.com's network in order to attract customers. That unleashed a powerful network effect that has helped to keep existing customers loyal while also attracting new users to the platform.

Given that online travel represented only a tiny fraction of the total travel market at the time, I realized that The Priceline Group still had a massive amount of room for future growth. I wound up buying shares on multiple occasions, and my average cost basis is around $385. With shares currently trading for more than $1,700 each, this stock has turned out to be one of my biggest winners. 

Despite its huge run, the Priceline Group shows no signs of slowing down. That makes it one of my highest-conviction stocks that I do not plan on parting ways with anytime soon.

Intuitive Surgical -- 6% of my portfolio

I was lucky enough to land a job in the sales and marketing department of a fast-growing medical device company right out of college. I wound up devoting more than a decade of my life to trying to convince healthcare providers to use a different product in their daily life. That turned out to be far more difficult than I thought it was going to be. The primary reason was that many providers are set in their ways and are very reluctant to try something new. That tendency gives the market-share leaders a massive competitive advantage over rivals.

Given that firsthand experience, I was blown away to learn what Intuitive Surgical has accomplished. This company pioneered the use of robots in the surgical setting with its groundbreaking da Vinci system. In just over a decade, this system has become the standard-of-care treatment for a number of procedures, including hysterectomies, prostatectomies, and more. That's helped drive sales of da Vinci systems ever higher, which has powered huge growth in the company's top and bottom lines.

ISRG Revenue (TTM) Chart

ISRG Revenue (TTM) data by YCharts.

I starting buying Intuitive's stock in 2010 and have added regularly to my position over the last few years. My average cost basis is now around $365. With shares currently trading for $740 each, the value of my investment has already more than doubled. That's turned this stock into my No. 2 overall holding.

Despite its past success, I'm still wildly bullish on this company's future. Intuitive's massive install base of 3,919 da Vinci machines gives it a huge leg up over potential rivals. The company is also investing heavily in new products and procedures that promise to further widen its moat. That should ensure that this company remains on top for decades to come, which is why I plan on sticking around for the long term.

Amazon.com -- 9% of my portfolio

Amazon.com was another stock that took me far too long to finally buy. While I was a happy customer for years, this company's nosebleed P/E ratio had long kept me on the sidelines. Even though I knew that e-commerce still had a long runway ahead of it, my value-investor tendencies kept me from becoming a shareholder.

That proved to be a monumentally stupid decision. I sat idly by as Amazon continued to take over the world. Shares continued to roar higher without me.

Thankfully, in 2010, I finally decided to bite the bullet and become an owner. I committed to a simple dollar-cost averaging strategy and purchased shares on 26 separate occasions over the next four years. Even though I was constantly buying the stock at 52-week highs, my average cost basis is only $190. With shares currently trading for $840 each, buying "high" actually proved to be a highly profitable decision.

Despite its massive size, Amazon continues to look poised for growth. Global e-commerce sales in 2016 were $1.9 trillion while Amazon "only" pulled in $136 billion in annual sales. That gives the company a worldwide market share of 7%. Meanwhile, the total e-commerce pie is expected to more than double over the next four years. I'm willing to bet that Amazon will continue to get its hands on more than its fair share of the gains.   

With plenty of room left for the business to grow, I could easily see myself remaining a happy shareholder for decades to come.