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."