Warren Buffett is well-known for saying that his preferred holding period for stocks is "forever," and it's not too surprising why. Once you've got your hands on a company that'll grow and grow for decades on end without interruption, selling it means missing out on future growth.

And that's why I plan never to sell this pair of businesses. There's always more money to be made by simply holding onto them, which takes zero effort. So without further ado, let's dive in and examine the two stocks I'll never let go of to see whether you might be interested in making a forever investment, too.

1. Innovative Industrial Properties

Innovative Industrial Properties (IIPR 0.37%) is a stock I'll never sell because I expect it to keep paying me money for holding it indefinitely. In case you aren't familiar, IIP is a real estate investment trust (REIT) that buys and rents out cultivation floorspace to cannabis companies. Specifically, it buys property from marijuana businesses already operating and then signs a lease with the former owners immediately after handing them their cash payment for the space. That simplifies the tenant search process as the prior owners are pinned to leases of between 15 and 20 years right off the bat.

And during that time, IIP and its shareholders reap the benefits of rent payments each and every month. In the last three years, its trailing-12-month cash from operations (CFO) popped by 404%, reaching an all-time high above $226.5 million. And the company is still cutting more deals to increase its number of properties every quarter.

The core reason I'll never sell this stock is its dividend, which currently has a forward yield of 6.1%. As the company continues to increase its dividend each year, the payout from my shares grows compared to what I paid for them. That incentivizes me to hold on to my shares forever because once the shares have paid for themselves, all the dividend payments thereafter are pure profit.

I also believe the cannabis industry's growth over the coming decades will ensure the REIT won't have any trouble finding new tenants or collecting rent from them in the long term. Even if cannabis is legalized, businesses will still need to raise capital through various methods, potentially by trading the properties they have on hand to IIP.

Likewise, as long as legalization isn't coordinated at the federal level, IIP fills a critical gap in the financing ecosystem for the cannabis industry: Traditional financial institutions like banks are leery of working with cannabis companies while the federal government's stance is still that of prohibition. So IIP is relatively prepared to keep flourishing for the foreseeable future, and I won't be selling it.

2. Costco

Costco Wholesale (COST -0.24%) is another stock I'll never sell, and here's why. As a Costco member, I frequently visit the company's warehouses to buy groceries and other consumer goods in bulk. However, the value of my purchases is negligible compared to the company's trailing-12-month revenue of $226.9 billion. Whenever I go to Costco, three consistent aspects of the experience are directly relevant to my investing-forever thesis.

First, the wholesaler offers lower prices than I can find elsewhere, and the products are of a decent standard of quality, so I always feel like I'm getting a good value. In other words, I have a financial incentive to shop there, which keeps me coming back -- and I know I'm not the only one.

Second, no matter what I purchase, I know I can easily return the product and get a hassle-free refund. So I'm incentivized to buy pricey items like TVs from Costco rather than a competitor to reduce the risk of financial harm if things aren't working as intended.

And third, if I really wanted to, I could probably buy close to 100% of the products I need to live from Costco, including prescription medicines, glasses, and even gasoline. They've expanded their selection by quite a bit in the 10 years I've been a member, and today you can even book a vacation or insurance package through them.

That's an important element of the bull thesis. It demonstrates that management is diligently seeking new product segments to make into drivers of long-term revenue growth and that the company's warehouses are credibly competing for the market share held by other grocers and consumer goods retailers.

Aside from the great customer experience I've had with the company, it's appealing to be a shareholder. Over the last 10 years, the stock has achieved a total return of 575% compared to the market's 238% gain.

The stock pays a dividend, currently with a forward yield of nearly 0.7%, and it occasionally disburses special dividends to return excess cash to investors. To sweeten the pot even more, it also does stock buybacks quarterly. The longer I hold it, the more money I'll make. So don't expect to read an article discussing how I sold my shares of Costco anytime soon -- or ever.