Every quarter, many of us in the investing world eagerly anticipate the SEC filings that the world's largest (and greatest) investment managers are required to file.

There is no single 13F that is more eagerly anticipated than that of Berkshire Hathaway's (NYSE: BRK-B) Warren Buffett.

The Oracle of Omaha isn't just one of the greatest stock pickers the world has ever seen. He is also one of the most selective. He seldom makes a significant new addition to his portfolio, usually instead preferring to add to existing positions or buy nothing at all.

So when he makes a big purchase of a new stock, it's kind of a big deal.

In the most recent quarter, Buffett revealed a rare new portfolio addition: Exxon Mobil (NYSE: XOM) -- for a cool $3.4 billion. Even for Buffett, that's a big purchase.

Investors have been scrambling to answer two questions: Why is Buffett was buying Exxon -- and why now? Buffett has been familiar with Exxon for decades, so what's changed about the company that merits a $3.4 billion investment from Berkshire?

It certainly isn't because Exxon's stock price has dropped: