Warren Buffett is best known as a wildly successful investor and one of the top three richest people in the world. Yet the billionaire doesn't use money as a measure of prosperity. For Buffett, the important metric is the relationships in his life — and his friend Bill Gates is taking notes.

According to Gates, Buffett's measure of success comes down to one question: "Do the people you care about love you back?"

In a blog posted Saturday, Gates says Buffett has helped broaden the way he thinks about his accomplishments.

Every year, Gates puts together his own end-of-year assessment, taking stock of his work and personal life. Thanks to Buffett, as well as Gates' wife Melinda, he says his assessment is very different than it was in his 20s, when he was first launching Microsoft.

"Back then, an end-of-year assessment would amount to just one question: Is Microsoft software making the personal-computing dream come true?" writes Gates.

Today, though he still assesses the quality of his work, Buffett and Melinda have inspired him to ask other questions about his life such as, "Did I devote enough time to my family?" "Did I learn enough new things?" and "Did I develop new friendships and deepen old ones?" Gates writes.