After years of criticizing airline stocks (he once referred to them as a "death trap for investors"), Warren Buffett's Berkshire Hathaway (NYSE: BRK-A) (NYSE: BRK-B) bought the big four airlines -- Southwest (NYSE: LUV), Delta (NYSE: DAL), American Airlines (NASDAQ: AAL), and United (NYSE: UAL) -- last year. Buffett was asked about it at the Berkshire Hathaway meeting, and he shared his reasoning. Here's our summary of that.

A full transcript follows the video.

This video was recorded on May 8, 2017.

Michael Douglass: Buffett was asked questions about the airline industry. Especially given the United incident, that's not surprising.

Gaby Lapera: Just for background, Berkshire owns --

Douglass: Four of the major airlines.

Lapera: Yeah, parts of the Big Four.

Douglass: Essentially, he and Munger -- it was funny; it's one of those things where two-thirds of their answers to questions about the airlines were saying it's not a great business. And then one-third was like, "Well, but here's why we decided to invest." It seems to me like they don't feel airlines are necessarily a home run or a grand slam, but they see it is a single-double kind of opportunity where you have -- passenger miles will increase over time, in the coming five to 10 years is their guess. If that's the case --

Lapera: Because of increasing globalization and people working all over the world.

Douglass: Yeah. If so, airlines should benefit, because they carry those people. Therefore, they bought the Big Four. And they bought the Big Four not because they thought they were necessarily the best positioned. They just wanted raw diversification and hadn't identified a single company that really seemed likely to disproportionately win.

Lapera: That's fair. Hedging your bets. Everyone does it.

Douglass: Yeah, including Warren Buffett.

Lapera: Including Warren Buffett.