He was unmarried (had a girlfriend, but that's different, as most married people will tell you). He never took vacations. In fact, he had been known for memorizing his employees' cars and license plates so that he could tell who left early and who stayed late each day.
So, it wasn't unusual that during 1991, a year on which the Fourth of July was a Thursday, that Gates's plan was to work on Friday, July 5. Four-day weekends weren't his thing.
But then, his mother kept bugging him. She and his father, Bill Gates Sr., were having lunch with a group of friends on Hood Canal in Washington, and among the guests would be Warren Buffett.
As Gates later told the story:
I didn't want to go. I told her I was too busy at work. Warren would be interesting, my mother insisted. But I wasn't convinced.
"Look, he just buys and sells pieces of paper. That's not real value added. I don't think we'd have much in common," I told her.
Eventually, she persuaded me to go. I agreed to stay for no more than two hours before getting back to work at Microsoft.
It was the beginning of a beautiful friendship, to steal a line from Casablanca. And, during the three decades since, Gates has described it as one of the most fortunate meetings of his life.
The multibillion-dollar financial decisions he's made recently are the direct result.
Last month, Gates announced he would give an additional $20 billion to the Bill and Melinda Gates Foundation, which he co-founded with his ex-wife, Melinda French Gates, in 2000.
The donations mean that Bill Gates and Melinda French Gates are now the biggest philanthropists in history, having given $55 billion to charity, with Buffett second on the list at $48 billion.
"This is going to supercharge ... basically all the work that we do," Bill Gates told Forbes in an interview at the time of the announcement. Gates still has a net worth of more than $111 billion, which makes him the fifth-wealthiest person in the world.
As Gates has told the story, his philanthropic efforts started with that meeting with Buffett in 1991, and then became more acute when his mother was found to have a rare form of breast cancer three years later and urged him to consider philanthropy.
After his mother died, Gates set up a $100 million foundation for his father to run after her death.
As The Wall Street Journal reported, after getting his son's OK on various donations, "Mr. Gates Sr. would then reply to all the grant seekers, sometimes including a $1 million check with little more than a single-page letter of congratulations."
In 2009, as he stepped down as CEO of Microsoft, Gates and his then-wife launched their foundation.
And in 2009, it perhaps became most serious after Gates and Buffett both attended a dinner with Chuck Feeney, a fellow billionaire (at least then), who had already begun an effort to give away almost his entire fortune during his lifetime.
Both Gates and Buffett have since described Feeney as their "hero," and said that the dinner meeting led them to announce the Giving Pledge, with 236 of the world's wealthiest people pledging to give at least 50 percent of their net worth to philanthropic causes.
I know that Gates has -- once again, I suppose -- become a controversial figure in some circles since the start of the pandemic, and the circumstances of his divorce.
But, I've often wondered whether other entrepreneurs-turned-billionaires wind up forming the kinds of relationships that Buffett and Gates began in 1991, and if, as a result, they've thought in the same ways about what the true legacy of their wealth might be after they're gone.
"My giving this money is not a sacrifice at all," Gates said at the time. "I feel privileged to be involved in tackling these great challenges, I enjoy the work, and I believe I have an obligation to return my resources to society in ways that have the greatest impact for improving lives. I hope others in positions of great wealth and privilege will step up in this moment too."