He just announced he would give nearly his entire fortune to the Gates' foundation starting now. Fortune's Carol Loomis asked him what his thinking was back in the 60's when he set up the Buffett Foundation, his answer is just so badass:
Well, when we got married in 1952, I told Susie I was going to be rich. That wasn't going to be because of any special virtues of mine or even because of hard work, but simply because I was born with the right skills in the right place at the right time.
I was wired at birth to allocate capital and was lucky enough to have people around me early on - my parents and teachers and Susie - who helped me to make the most of that.
In any case, Susie didn't get very excited when I told her we were going to get rich. She either didn't care or didn't believe me - probably both, in fact. But to the extent we did amass wealth, we were totally in sync about what to do with it - and that was to give it back to society.
In that, we agreed with Andrew Carnegie, who said that huge fortunes that flow in large part from society should in large part be returned to society. In my case, the ability to allocate capital would have had little utility unless I lived in a rich, populous country in which enormous quantities of marketable securities were traded and were sometimes ridiculously mispriced. And fortunately for me, that describes the U.S. in the second half of the last century.