“An embarrassing amount of how well you do, particularly in your twenties, has to do with attitude,” Jassy said in an interview with LinkedIn CEO Ryan Roslansky. 

After giving up on a career in sportscasting and then dabbling in music management, Jassy—who took Amazon’s helm after Jeff Bezos stepped down in 2021—joined the tech giant in 1997 as a marketing manager. The Harvard Business School alumnus went on to launch the cloud-computing platform Amazon Web Services with Bezos. By 2020, AWS generated $13.5 billion of Amazon’s $22.9 billion in annual operating income, making him a clear candidate for CEO. 

Looking back, he told Roslansky that “there’s so many things that you can’t control in your work life, but you can control your attitude.” 

“I think people would be surprised how infrequently people have great attitudes,” he said. “I think it makes a big difference.”

The reason why Jassy thinks positivity determines success is pretty straightforward: People want to be around positive people. 

“You pick up advocates and mentors much more quickly,” he added. “People want those people to succeed—and it’s very controllable.”

But a cheerful disposition alone won’t get you far, Jassy warned. Aspirational workers need to follow through on a can-do attitude with action.

“Do you work hard? Do you do what you said you were going to do?” he asked.

Enthusiasm musn’t end with your twenties

It’s easy to assume that once you’ve climbed the ranks, it’s now your turn to impart knowledge onto others. But Jassy warned that your enthusiasm for learning mustn’t end in your twenties—or when you reach management.

“The biggest difference between the people I started with in my early stages of my career and what they’re doing now has to do with how great they were at learning,” he said.

“There are some people who get to a certain point and it almost feels threatening to them to learn,” Jassy pointed out, adding that it can highlight gaping holes in your knowledge and make you question whether you’re worthy of your rank. 

“But the second you think you know it all is the second you’re really starting to unwind.”

It’s why he’s a fan of a squiggly career that lets you stretch your abilities and try on many different roles and industries for size.

“Along the way, you’ll keep picking things up if you let yourself,” Jassy concluded. “You’ll wind your way around something that you’re really good at.” 

Success starts on the school field

Before entering the world of work, Jassy revealed, he was an aspiring athlete. The multimillionaire told Roslansky that he spent “a lot” of his school time on the field playing “all the sports,” instead of keeping his head down and studying. 

“I had this delusion that maybe I could be a professional athlete,” he revealed. “I played that national tennis circuit and then I played soccer competitively—and of course, I wasn’t good enough to be a professional athlete, so I’m not.”

Although Jassy failed at realizing his sporting dreams, the success he enjoys today could be thanks, in part, to the years he dedicated on the pitch. An extensive study of U.S. Ivy League alumni has shown those who are consumed with sports—not books—in their youth go on to become more successful than their peers later on in life.

The researchers found that sporty students were significantly more likely to land finance or business-related jobs after college, with more senior positions and high salaries than nonathletic students.

Jassy might not have known it at the time, but the research suggests that while dedicating his spare time to competing in soccer—which often relies on teamwork, communication, and accountability—he was banking skills fit for the boardroom.

A version of this story was originally published on Fortune.com on May 20, 2024.