Bill and Melinda Gates have explained why they're giving away their fortune

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

As part of that effort, they pledged to give most of their wealth to charity during their lifetime or to commit to doing so 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 in hopes of improving healthcare and bringing people out of poverty across the world, among other things.

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

According to Bill Gates, 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," he wrote in the letter. "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."

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.

The Gates' 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 Gates takes a few sips from a device that turns feces into drinking water.)

"At Microsoft, I got deep into computer science," Bill Gates said in the letter. "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 Gates 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 18 years," Melinda Gates wrote. "That's the majority of our marriage. It's almost the entirety of our children's lives. We do the work because it's our life."