It's a chance to "effectively prioritize my commitment to addressing some of the world's toughest challenges," Gates wrote.
This is a big announcement, and it's a long time in coming. In fact, while I know this will sound very specific, you can trace his decision back to a specific moment: July 5, 1991--and the lunch Gates wound up having that day.
The big transition
To understand, it helps to realize that Gates passed a huge milestone recently--yet, an oddly unrecognized one.
Of course, Gates stayed involved with Microsoft: He was the chairman of the board until 2014. And he joined the board at Berkshire Hathaway.
But in the meantime, he was making an enormous, nearly unprecedented transition.
In short, he's one of the few people in history--certainly during the 20th and 21st centuries--who reached the absolute pinnacle of one field (tech/business/entrepreneurship) only to step aside, and then climb the pinnacle of another field (philanthropy).
But, the roots go back to the 1991 lunch. It was Gates's parents who invited him to it.
He didn't want to go at first, even though a very high-profile person was going to be there (a mutual friend of his parents had introduced them), and his parents insisted he was someone their son would want to meet.
As Gates told the story in 2005:
It was a funny event because my mom's very sociable, always getting people together. I at this time didn't believe in vacations, was totally focused on my job. So when she said to me, "You've got to come out and meet Warren...," I said, "Mom, I'm busy!"
As in, Warren Buffett.
Gates picked up the story in a 2016 blog post, remembering that he told his mom:
"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.