One of this year's hottest stocks it only getting hotter. Roku (NASDAQ:ROKU) hit new all-time highs last week. The streaming video pioneer has treated investors to a 239% return this year, one of the very few stocks that have more than tripled in 2019.

Roku's doing a lot of things right, naturally. A stock doesn't deliver a decade of gains in less than six months by accident. Let's go over the reasons why Roku has become a market darling for growth investors in 2019.

1. There's momentum to the model

Everything seems to be going right for Roku these days. There are now more than 29 million active users on the platform, 40% more than it had a year ago. Those binge viewers are streaming roughly 3 billion hours of content a month, another metric that's growing nicely on a per-user basis. 

Roku's improving monetization from its engaged audience is paying off. Average revenue per user has climbed 27% over the past year, and that stacked on top of its growing base of video buffs is why Roku's platform revenue skyrocketed 79% through the first three months of the year. 

2. Starting lines matter

Stocks aren't always hot, and often one year's biggest winner was one of last year's more disappointing laggards. Sandbagging can do a stock wonders, and Roku fits the bill. The shares plunged 41% through 2018, giving it a lot of upside heading into this year.

Roku is still a rock star even if we account for last year's brutal slide. The stock has doubled since the start of last year. However, there is a big difference between a 101% pop since the end of 2017 and having more than tripled when the starting line is the end of 2018.

3. Analysts are buying into the growth story

It's easy to hate on a stock that's out of favor, and last year the narrative during Roku's slide was that it was just a small fry competing against tech giants. The story has turned this year, and now it's an advantage for Roku to not be one of the world's most valuable tech companies. Roku's nimble ways and its agnosticism are the reasons it's landing deals with smart TV manufacturers to be the operating system of choice, breathing new life into its streaming sticks as well. 

Analysts sometimes shy away from stocks that have risen too high too soon, but Roku is still converting the skeptics. Michael Morris at Guggenheim upgraded the stock from neutral to buy earlier this month, encouraged by the core trajectory behind its blowout first quarter and the hearty secular tailwinds benefiting the industry. Between the migration of cord-cutters to streaming platforms and ad rates continuing to close the gap with traditional broadcasts, Roku's growth should be pretty impressive through the next few years.  

Not every Wall Street pro is huffing and puffing to keep up with the rising stock price. Kyle Evans at Stephens downgraded the stock last month on valuation concerns. However, that has been the exception instead of the rule. Analysts aren't getting queasy about Roku's monster gains this year, and that's a good sign for the stock's near-term prospects.  

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.