The philanthropists have long been close friends with Warren Buffett. Their letter both outlines what the Bill & Melinda Gates Foundation has done with Buffett's donations and also celebrates their friend's positive outlook.
"Optimism is a huge asset," Melinda says in the letter. "We can always use more of it. But optimism isn't a belief that things will automatically get better; it's a conviction that we can make things better.
"We see this in you, Warren. Your success didn't create your optimism; your optimism led to your success."
"Ten years ago, when we first got word of your gift to the Foundation, we were speechless. It was the biggest single gift anyone ever gave anybody for anything," Bill and Melinda write.
And while Buffett has given away a fortune, the CEO and chairman of Berkshire Hathaway is famously frugal. He never spends more than $3.17 on his breakfast, for example.
In the letter, Gates tells the story of a particularly thrifty lunch Buffett took him out for years ago.
"Remember the laugh we had when we traveled together to Hong Kong and decided to get lunch at McDonald's? You offered to pay, dug into your pocket, and pulled out … coupons!" writes Bill.
The extensive letter that follows outlines the Foundation's investment in the developing world, providing vaccines, contraception and other resources for the poor.
In the last 25 years, the number of people living in extreme poverty in the world has fallen by half, Bill writes.
"We're trying to channel your empathy, add your optimism, mix in technology, apply strategy, and work with partners to save more lives. That's how we're spending your gift," says Melinda.
"It won't surprise you to know that we're more optimistic than ever," says Bill.
The Bill & Melinda Gates Foundation has ambitious goals. It aims to wipe out malaria, TB, HIV, malnutrition and preventable deaths.
Thanks to people like Buffett, Bill and Melinda write, "the future will surprise the pessimists."