There have been several recessions over the last half-century, but none as bad as we've experienced over the last four years.

Why?

There are dozens of reasons, but the simplest and most powerful is that, after three decades of leveraging up, U.S. households took on too much debt, and the bill came due. The fact that Bank of America (NYSE: BAC) and Citigroup (NYSE: C) went from two of the most profitable companies in the world to the brink of bankruptcy in a matter of months shows how dramatic and fast that shift came.

But why did households go into so much debt to begin with? I recently sat down with Mesirow Financial chief economist Diane Swonk, who offered some interesting insights when I asked her why this recession was different than those in the past. Have a look:

Fool contributor Morgan Housel owns B of A preferred. Follow him on Twitter @TMFHousel. The Motley Fool owns shares of Bank of America and Citigroup. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.