"Optimism is a huge asset," Melinda Gates writes in this year's Bill & Melinda Gates Foundation annual letter addressed to Berkshire Hathaway CEO Warren Buffett.
"We see this in you, Warren. Your success didn't create your optimism," she writes. "Your optimism led to your success."
In her book "Nine Things Successful People Do Differently," Heidi Grant Halvorson, a social psychologist and associate director of the Motivation Science Center at Columbia University notes the difference between realistic optimists, like Buffett, and unrealistic optimists.
"To be successful, you need to understand that vital difference between believing you will succeed and believing you will succeed easily," Grant Halvorson writes.