"I had to be a little careful not to try and apply my standards to how hard they worked," he said. "I knew everyone's license plates so I could look out in the parking lot and see when did people come in, when were they leaving. Eventually I had to loosen up as the company got to a reasonable size."

Gates, who dropped out of Harvard University at 19 to start Microsoft with Paul Allen in 1975, stepped down as the CEO of the software company in 2000 and stepped down as the company's chairman in 2014. He is now co-chair of the Bill and Melinda Gates Foundation.

He continued: "In the early years, the intensity had always been about the project, and so then (when) Steve got sick, it was far more mellow in terms of talking about our lives and our kids. Steve was an incredible genius, and I was more of an engineer than he was. It was fun. It was more of a friendship that was reflective."