Some entrepreneurs believe that life is too short to deal with difficult customers. Misapplying the 80-20 rule, they try to dump the 20% of customers who cause 80% of their problems. This is a huge mistake. Before launching Bluecore, I wish I had known that my most skeptical, demanding customers would become our best advocates.

This ode to difficult customers may seem counterintuitive to entrepreneurs with “happy ears” syndrome. Early on, it feels good to find validation, right? You pitch some prospective customers and they say, “Wow, that’s a great idea!” Forget the naysayers—you must be on the right track. Too bad those prospects will not buy your product six months later. They’re being nice, just like Aunt Carol who said, “I love your startup idea, honey! Want some more eggnog?”

See also: Mogul CEO: It’s Time to Start Setting Higher Expectations

Before building the Bluecore platform in 2013, I had the good fortune to land a meeting with a digital marketer we’ll call “Jake.” He worked for an outdoor retailer and took the meeting because he had strong opinions about triggered email technology. I described what we planned to do. Jake essentially said it was nice, but that we needed two other pieces of core functionality before he’d become our customer.

Four months later, I returned to Jake with our complete product, and it still wasn’t good enough. He asked for more. In the meantime, we signed our first 10 customers. “Well they like the product,” I thought to myself. “Should I keep building for Jake?”

I valued Jake’s opinion too much to ignore it, because while his demands were tough, his ideas were exceptional. We kept building for him, and eventually Jake did become a customer. He forced us to build a much better product than we would have built otherwise.

The experience made me realize that startups are like high school students: They need at least one hard-ass teacher to grow. Instead of spewing half-hearted encouragement, this teacher dishes out critique and lower grades. The students think the teacher must hate them, when in fact he wants them to excel. Many students accuse the teacher of being unfair, but a few use the critique to up their game. Those few become the top performers in whatever they do.

It is dangerous to dismiss your most demanding customers. Ten months ago, I almost let go of our most difficult account at the time because it was consuming an inordinate amount of resources. A player in the sharing economy, this customer wanted us to build a segmentation product ASAP—and brought it up constantly. We reached our boiling point, but then recognized that their demand was spot on. We built the segmentation product, and this company replaced its entire marketing stack with our platform. It’s scary to think that we almost fired a customer whose feedback led to a product that will account for half of our revenue in 2016.

You might think your skeptical, difficult customers will bash you if they have the chance. Not so. While we were raising our Series B this year, an investor asked to speak with one of our toughest customers, a Fortune 500 clothing marketer. I was certain we’d lose this investor, but I had to make the intro anyway. This marketer gave Bluecore the strongest recommendation we had ever received. Yes, she also talked about our shortcomings, but that’s what made her endorsement so credible and effective. The investor joined our B round.

Here’s the key point: Don’t confuse critique with disappointment. Customers are demanding because they want you to deliver at the top of your game, and they’re skeptical because they care about your product and its capabilities. Be wary of the customers who have only nice things to say.

Despite our success over the past two years, I wish I had known all of this before launching Bluecore. My go-to market strategy would have been to seek out the most difficult, skeptical customers I could possibly find. I would have turned to them for references and product development feedback early and often. I would have spent more time with people like Jake who could rip our ideas to shreds.

Ultimately, business is about attracting and keeping customers. Your toughest critics will help you accomplish both.

Read all responses to the Entrepreneur Insider question: What’s something you wish you knew before starting your business?

The Biggest Challenge Every Entrepreneur Faces by Jeff Ruby, founder and CEO of Newtopia.

Here’s What Happens When Your Company Only Focuses on Data by Allison Berliner, founder and CEO of Cataluv.

What Every Entrepreneur Should Be More Prepared for by Feris Rifai, cofounder and CEO of Bay Dynamics.

Why Virtual Offices Don’t Work by William Vanderbloemen, founder and CEO of Vanderbloemen Search Group.

The One Quality That Defines a Great Entrepreneur by Anthony Katz, founder of Hyperice.

What Every Entrepreneur Can Learn From Apple by Michael Maven, founder of Carter & Kingsley.