What goes up must come down. Except when it doesn't.

It's easy for investors to worry about buying a stock that's gone up a lot. Concerns can arise that maybe the momentum will fade after achieving a big gain. Phrases such as "reversion to the mean" begin coming to mind.

However, many stocks that soar keep right on soaring. The law of gravity might apply in the physical world, but it doesn't always hold true in the investing world. Here are three unstoppable stocks at all-time highs that you can still buy.

Red line with arrow trending up with a dollar sign drawn above the line and a man wearing a suit standing on a ladder holding his hand up to the arrow

Image source: Getty Images.

1. Apple

Apple (NASDAQ:AAPL) stock vaulted 81% higher in 2020 even after a major decline in the coronavirus-caused market sell-off early in the year. The tech giant now sports a market cap north of $2.3 trillion. I think Apple could grow even larger in 2021.

I'm in good company. Wedbush analyst Daniel Ives recently raised his price target for Apple to $175, a premium of 22% over its current price. Ives even anticipates that the tech stock could even deliver a 36% gain over the next 12 months with its share price surging to $225. My view that even if his numbers are too rosy, the underlying logic behind his optimism is sound.

Demand for Apple's new 5G-enabled iPhones continues to be very strong. Ives called the potential growth for the new versions of the smartphone "eye-popping." As the iPhone goes, so goes Apple. The company's ecosystem that includes the App Store, Apple Music, and iCloud, largely revolve around the iPhone.

Sure, Apple stock is pricey with shares trading at nearly 35 times expected earnings. Barring an overall stock market downturn, though, my hunch is that this premium valuation won't get in the way of Apple's momentum. 

2. PayPal Holdings

PayPal Holdings (NASDAQ:PYPL) was an even bigger winner than Apple last year, racking up a gain of nearly 117%. The COVID-19 pandemic fueled a sharp increase in online shopping, which helped boost the fintech company's sales. My view is that the fun is just getting started for PayPal.

Could the roll-outs of coronavirus vaccines cause e-commerce growth to slow? I don't think so. There's still a huge growth runway. In the third quarter of 2020, e-commerce accounted for only 13.5% of total retail sales in the U.S. PayPal is also expanding beyond e-commerce -- its apps now support QR functionality for buying products in brick-and-mortar stores.

The company's move into cryptocurrency could also spur revenue growth. PayPal users can trade cryptocurrencies as of late 2020. BTIG analyst Mark Palmer thinks the company's revenue could increase by over $1 billion by 2022 from its cryptocurrency services. PayPal plans to also offer cryptocurrency features in its Venmo peer-to-peer payment app.

Speaking of Venmo, it's also a key growth driver for PayPal. The company continues to expand Venmo's functionality, recently launching the ability for users to cash checks with the app. With the rise of e-commerce and digital payments, I think that PayPal is an unstoppable stock that's poised to soar yet again in 2021.

3. Teladoc Health

Teladoc Health (NYSE:TDOC) stands out as another company that benefited tremendously from the COVID-19 pandemic. The healthcare stock skyrocketed 139% in 2020 and is up more than 40% year to date.

It makes sense that the coronavirus outbreak fueled increased adoption of telehealth. Patients preferred virtual visits with their healthcare providers instead of going into offices where their chances of getting COVID-19 were higher. Teladoc's visits skyrocketed -- and so did its revenue.

Don't think that telehealth will go away once coronavirus worries do, though. More employers, health plans, and individuals were able to see the value of telehealth during the pandemic. In fact, consulting firm McKinsey & Company projects that the U.S. telehealth market could approach $250 billion after the pandemic is over. 

Teladoc has a huge growth opportunity in just increasing usage at its current clients. At the end of the third quarter of 2020, the company had around 73 million members. There are another 65 million potential users at current Teladoc clients. I expect the company will gain ground with these users as well as add significantly to its client base in 2021 and beyond. And I'm confident that Teladoc will set many more new highs over the coming years. 

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.