Krish Himmatramka's engagement ring company, Do Amore, sounds like any investor's dream: Its diamond products are ethically sourced and thoughtfully produced, and the business makes a lot of money.

The company brought in $11.5 million in 2021 revenue, eight years after launching in 2013. But on Friday's episode of ABC's "Shark Tank," Mark Cuban rejected Do Amore for one crucial reason: He foresaw competition, and didn't think Himmatramka's company could stand up to it.