Netflix (NASDAQ:NFLX) stock is up by more than 350% over the last five years, making the online streaming leader one of the most profitable positions in my personal portfolio. Big gains are nice to have, but past performance is no guarantee of future results, so investment decisions should always be analyzed with a forward-looking perspective.

In that spirit, here are the main three reasons I own Netflix stock.

Competitive strength
Even if a company is delivering spectacular financial performance, competitive strength is always a crucial factor to consider. Success attracts the competition, and players such as Amazon (NASDAQ:AMZN), Time Warner (NYSE:TWX), and Apple (NASDAQ:AAPL) are major competitive threats to Netflix.

Amazon offers its Prime Video service as part of its Amazon Prime membership program. For $99 per year, Amazon Prime members get free two-day shipping, Prime Video, Prime Music, and access to Amazon Kindle Owner's Lending Library. This is quite a competitively priced proposition, and Amazon is building a huge library of content with over 40,000 titles available for streaming. In addition, Amazon is expanding into original programming with successful productions such as Transparent.

Netflix management considers Time Warner's HBO its most relevant competitor, and for good reason. HBO is home to many of the most popular series around, including the massively successful Game of Thrones. Time Warner has recently launched HBO Now as a direct steaming service, pricing the subscription at a premium price of $14.99 monthly. The company is also expanding into Latin America, offering HBO Go as a stand-alone online subscription service in the region, so Time Warner seems quite serious about its plans to join the streaming revolution on a global scale.

Apple is reportedly working on its own exclusive video programming, and management has repeatedly said that Apple has ambitious plans in online TV. Apple is perhaps the most powerful tech company in the planet, and it owns enormous strategic and financial resources, so investors in Netflix may want to keep a close eye on Apple and its intention to gain ground in online streaming.

Amazon, Time Warner, and Apple are just three relevant examples to consider -- the main point is that Netflix will be facing increased competitive pressure over years to come, so you should not invest in Netflix unless you believe the company has enough competitive strength to continue thriving in a more challenging environment.

Fortunately, content is king in the industry, and Netflix has proven time and again that it knows how to deliver what customers want. Original productions such as House of Cards, Orange Is the New Black, and Narcos provide customer value and competitive differentiation. Besides, Netflix is competitively priced, at only $8.99 monthly for its standard service.

Netflix is a well established leader in online streaming, and the company has access to enormous amounts of data about the preferences and viewing habits of its members. This is a crucial strategic advantage when it comes to buying or creating successful new content.

Conquering the world
Netflix ended the third quarter of 2015 with a total of 69 million members around the world, 42 million of them in the U.S., and the remaining 26 million in international markets. Growth will probably slow down in the U.S., but international markets should remain powerful growth drivers in the middle term.

Data source: SEC filings. 

The company still has a lot of room for international expansion; Netflix has recently entered Italy, Spain, and Portugal, and management is planning to venture into South Korea, Hong Kong, Taiwan, and Singapore in early 2016. According to data from the Australian Communications and Media Authority, Netflix is off to a strong start in Australia -- the company has reached over 2.5 million users in the country in the first half of the year.

Netflix is increasingly becoming a global growth story, and this should drive powerful revenue growth in the coming years.

Improving profitability
Content is expensive, but revenues are outgrowing costs, and Netflix is producing expanding profit margins in the U.S. The company reported a contribution margin of 32.4% of sales in the third quarter of 2015, a significant increase from 28.6% of revenue in the same period last year. Management is targeting a contribution margin of 40% in the U.S. by 2020.

Global expansion will weigh on margins over the middle term, but management is forecasting break-even results in international markets by 2016. The company expects international profitability to increase in 2017 and beyond.

It looks like Netflix is well positioned to continue producing strong revenue growth and expanding profit margins in the coming years, and for this reason, I'm not planning to sell my Netflix stock anytime soon.

Andrés Cardenal owns shares of Amazon.com, Apple, and Netflix. The Motley Fool owns shares of and recommends Amazon.com, Apple, and Netflix. The Motley Fool recommends Time Warner. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.