With a

net worth of more than $90 billion, Microsoft

co-founder

Bill Gates

is the second richest person in the world, behind only

Amazon

founder

Jeff Bezos, according to Forbes

. He has so much money, in fact, that it has become a full-time job for him, his wife, Melinda, and 1,500 employees of the

Bill and Melinda Gates Foundation

to give it all away.

So why do the Gates work so hard to give their billions to philanthropy? In the couple's

10th annual letter

describing their foundation's charity work released Tuesday, the billionaires opened up about the subject.
There are two reasons to do something like this," the letter says, referring to the $4 billion a year the foundation spends in developing countries trying to end child mortality, distribute vaccinations and improve access to education, plus another $500 million it shells out in the United States.

First, "it's meaningful work," according to the letter.

"Even before we got married, we talked about how we would eventually spend a lot of time on philanthropy," writes Bill. "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."

To that end, the Gates have also co-founded

The Giving Pledge

with their billionaire buddy

Warren Buffett

. The platform invites billionaires to commit to giving away the majority of their money to charity.

It's an enjoyable challenge

For the Gates, the endeavor is also a pleasure. "We have fun doing it. Both of us love digging into the science behind our work," writes Bill. Whether helping to eradicate polio or breed high-yield

"super cows,"

often, deciding where to allocate money requires they study biology, chemistry or agronomy, he says in the letter. "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.

"It's rare to have a job where you get to have both a big impact and a lot of fun. I had it with Microsoft, and I have it with the foundation," writes Bill. "I can't imagine a better way to spend the bulk of my time."

here is a specific need

In the letter, the Gates acknowledge the

extreme wealth inequality

their fortune represents. "No, it's not fair that we have so much wealth when billions of others have so little," says Melinda. "And it's not fair that our wealth opens doors that are closed to most people."

So, "If we think it's unfair that we have so much wealth, why don't we give it all to the government?" asks Bill. "The answer is that we think there's always going to be a unique role for foundations."

philanthropic foundations can "take a global view to find the greatest needs, take a long-term approach to solving problems, and manage high-risk projects that governments can't take on and corporations won't. If a government tries an idea that fails, someone wasn't doing their job. Whereas if we don't try some ideas that fail, we're not doing our jobs," says Bill.

It's how they were raised

"We both come from families that believed in leaving the world better than you found it," writes Melinda. "My parents made sure my siblings and I took the social justice teachings of the Catholic Church to heart. Bill's mom was known, and his dad still is known, for showing up to advocate for a dizzying number of important causes and support more local organizations than you can count.

"Our goal is to do what our parents taught us and do our part to make the world better," says Melinda.

After working for the foundation full-time for 18 years, "By now the foundation's work has become inseparable from who we are. We do the work because it's our life," says Melinda.

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