" We see this in you, Warren . Your success didn't create your optimism; your optimism led to your success."

"Optimism is a huge asset," Melinda Gates writes in the Bill & Melinda Gates Foundation's annual letter , addressed, this year, to Buffett. "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.

According to his friends and fellow billionaires, Bill and Melinda Gates, the secret to the octogenarian's success is his unceasing optimism.

No matter where things are right now, he knows in the long run they're getting better.

Buffett's deep conviction that the world will be a better place in the future than it is today has guided his investing strategy.

"He's optimistic about the country, about the future and about you," the Gates write in an op-ed published on CNN.com earlier this week. "No matter where things are right now, he knows in the long run they're getting better. You doubt it? Don't bet against it; he's made billions investing in that belief."

As if Buffett's billions of dollars aren't proof enough, scientific research says that optimism leads to success, too.

"Optimism changes subjective reality," says cognitive neuroscientist Tali Sharot in a popular TED talk on the topic. "The way we expect the world to be changes the way we see it. But it also changes objective reality. It acts as a self-fulfilling prophecy. Controlled experiments have shown that optimism is not only related to success, it leads to success."

There's no guarantee that optimism will bring you billions, as it has for Buffett — but it certainly can't hurt.