Bill Gates reveals the 2 reasons why he's giving away his $90 billion fortune

• The Bill and Melinda Gates Foundation has released its annual letter.
• In the letter, Bill and Melinda say that they give away their money because doing so is both meaningful and fun.
• The pair says they do the work "because it's our life."

In 2010, Bill and Melinda Gates started The Giving Pledge along with friend (and fellow billionaire) Warren Buffet.

As part of that effort, they pledged to give over half their wealth to charity during their lifetime or after death. The Gates family has been making good on this pledge for nearly two decades through The Bill and Melinda Gates Foundation, which donates money to improve healthcare and cut down on poverty across the world, among other things.

In the foundation's annual letter, released this week, Bill Gates explains why he and Melinda are giving away the Microsoft fortune.

According to Bill, it comes down to two reasons: the work is both meaningful and fun.

"Even before we got married, we talked about how we would eventually spend a lot of time on philanthropy. We think that's a basic responsibility of anyone with a lot of money. Once you've taken care of yourself and your children, the best use of extra wealth is to give it back to society," he wrote in the letter.

Melinda attributes her interest in philanthropy to her upbringing in the Catholic Church, which taught her to value social justice. "We both come from families that believed in leaving the world better than you found it," she wrote.

Bill and Melinda's love of the work is easy to see in the way they discuss their various initiatives on Facebook and in the Gates Notes blog. (The classic example of this is the video in which Bill takes a few sips from a device that turns feces into drinking water.)

"At Microsoft, I got deep into computer science. At the foundation, it's computer science plus biology, chemistry, agronomy, and more. I'll spend hours talking to a crop researcher or an HIV expert, and then I'll go home, dying to tell Melinda what I've learned," Bill wrote in the letter.

Bill has said in the past that his children won't be billionaires since he's giving so much money away - but that they are heavily involved in the foundation.

"Bill and I have been doing this work, more or less full-time, for 17 years. That's the majority of our marriage. It's almost the entirety of our children's lives," Melinda wrote. "We do the work because it's our life."

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