Roku (NASDAQ:ROKU) keeps rewarding bullish investors. Shares of the streaming video pioneer kicked off the new trading week by hitting another all-time high on Monday. Roku stock has now more than doubled since the end of March, more than quadrupling since going public just 11 months ago.

There's a lot of love and hate for Roku. Bulls are loving the high ceiling for Roku's platform revenue as more viewers flock to its service-agnostic streaming operating system. Bears feel that Roku was overpriced when it hit the market last September, so naturally they're even more adamant about the valuation with the stock quadrupling in less than a year. The bulls are getting the upper hand now, so let's go over why Roku is rocking its way up to new all-time highs this week.

Roku's operating system on a Windows platform.

Image source: Roku.

1. It's hard to bet against accelerating growth

Roku didn't seem like a scorcher when it hit the market last year. Its prospectus showed that Roku grew its revenue at a modest 22% clip in 2016, improving slightly to 23% through the first half of last year. Things were just starting to heat up. 

Revenue would go on to accelerate through the latter half of last year, Roku's first two quarters as a public company. Roku keeps putting the pedal to the metal in 2018, growing its top line by 36% in the first quarter and a blistering 57% clip in its blowout second quarter.

The secret to Roku's acceleration -- it's growing its revenue at more than twice the rate of where it was when it went public a year ago -- is the transformation of its model. Stagnant hardware revenue continues to shrink as a top-line contributor with the faster growing and higher-margin platform revenue taking the wheel. 

2. Engagement and monetization are everything

Platform revenue nearly doubled in Roku's latest quarter. There are now 22 million active users on Roku's operating system through either its namesake media players or the growing number of smart TVs that come preinstalled with Roku's navigational software, 48% ahead of where it was a year earlier.

The story gets better. Roku's users consumed 5.5 billion hours of content through the platform, 57% higher than a year earlier. It's not a shock to see folks leaning more on digital content, but seeing usage outpace account growth confirms improving engagement levels. Then we get to a 48% improvement in average revenue per user, outpacing the usage growth per user. In short, monetization is also on the rise. 

3. Shorts are getting squeezed out

There are a lot of people betting against Roku, but not as many as there were a couple of months ago. There were more than 7.5 million shares sold short as of mid-August. The number of shares betting against Roku peaked at more than 9.9 million by the end of February. Roku stock would bottom out below $30 just a few weeks later. 

It's easy to see why the shorts got nervous after a decent first quarter, but that report wasn't immediately well received by investors. It got harder to justify being short Roku earlier this month when it hit it out of the park with its second-quarter performance. The same bearish thesis on Roku from a year earlier no longer applies, and with the model morphing and growth scorching, it's a big risk to bet against a stock that has more than doubled over the past few months.