It was 1973, and Koch had just completed his second year of Harvard Business School’s JD/MBA program. He said the program allowed students to do law school and business school at the same time, and often led to a career as a corporate lawyer or working for a big company. But by the end of his second year, Koch was questioning if that path was truly for him.

“I’ve been going to school since I was five years old,” Koch said of the dilemma in an interview with Fortune.  “I’ve never really done anything in the real world and yet, I’m on this path leading me to a place I’m not sure I want to be.”

His push to explore the world and get hands-on experience ended up leading him on the path to founding Samuel Adams. But now looking back, Koch realized that one crucial skill he learned while building the company isn’t even found in a textbook at Harvard.

How to sell a company 

Koch’s path towards success didn’t start in the classroom. In fact, he said that the selling skills he learned aren’t even taught at Harvard. The Ivy League institution offers students courses in marketing, sales management, and business analytics, yet a course on basic selling tactics is nowhere to be found. 

“Selling is this really, really important skill that business schools don’t teach to this day,” Koch said.

It’s a skill he had to learn on the ground—literally. When Koch launched Samuel Adams in 1984, he couldn’t find a distributor who would agree to sell his beer. Which meant he had to sell it himself.

Armed with a family recipe and a dream, the then 34-year-old set out walking door-to-door with a briefcase of beer, hoping to convince bar owners and managers to stock the brew. In one briefcase, he could fit seven beers, two ice packs, and a sleeve of cups for sampling. Koch says he had “about a 5% success rate,” meaning for every 20 bars he visited, he’d open one new account. 

“If I didn’t go from bar to bar with the cold beer in my briefcase and get people to carry it, I was going to go broke really quickly,” he said.

The determination and door-knocking paid off: Samuel Adams launched with an investment of $140,000 and two employees before skyrocketing into a $3 billion business over the course of 40 years. The decades-old brand is staying modern too: Koch expanded his alcohol empire under the umbrella of Boston Beer Co. to include bar favorites like hard seltzer brand Truly, hard iced tea brand Twisted Tea, and hard cider brand Angry Orchard.

Now, four decades later, Koch is sharing his biggest business lessons, and wants entrepreneurs to understand the importance of one core function that may seem obvious: selling. 

“Nobody goes to college because they want to be a salesman,” he said. “It has a negative connotation.”

Citing pop culture classics like The Wolf of Wall Street and Death of a Salesman, Koch explained that in pop culture, particularly among white-collar and educated workers, “salespeople are portrayed as kind of sleazy.”

“But what I learned is, done right, it’s a very noble activity,” he said, adding that exemplary salespeople demonstrate how their products can help consumers with their daily lives and goals. Conversely, salespeople shepherding “crappy,” non-beneficial products are “charlatans.” 

“People are too smart to be fooled like that,” he said. “So to me, selling is not only necessary but noble.”

Success is a mindset

Koch’s path to building his empire wasn’t just hard work, he says: humility was a key building block. He says it’s one of the best pieces of business advice he ever received—and it came from his grandmother before he departed for Harvard.

She reminded him: “Jim, remember humility is a virtue.”

Approaching business with humility and gratitude for the success that you already have will lead you to a happy and rewarding life, Koch said.

Overall, success didn’t come Koch’s way traditionally. The entrepreneur used hands-on training from running wilderness courses to become a leader, taught himself how to sell when Harvard didn’t, and kept his grandmother’s words on humility in his head as his success grew. 

Read more from Fortune

Now, Koch helps pass on what he’s learned to entrepreneurs through his Brewing the American Dream program. 

“I believe that my job as a businessperson is to try to pay forward, share the wealth, however you want to say it, because at the end of the day if you’re the only person who benefits from your success, you’re not going to have much of it.”

The program helps entrepreneurs with two things Koch didn’t have access to when he started Samuel Adams: loan money and coaching and counseling advice. While partnering with the ACCION opportunity fund, the program makes loans to businesses that no bank is going to touch. Koch says they look at the passions of the entrepreneurs and the quality of their products.

He says the program has a repayment rate of about 98%. And since then 2008, they’ve made $113 million in loans to 4,500 companies, provided coaching and counseling to around 16,000 people, and those companies have saved or created almost 11,900 jobs in their communities.