So, I did a bit of digging into this to find a reasonable explanation for this one. I've gone through dozens blog posts, research papers, and skimmed through two webinars on the topic. Turns out, this is actually a pretty contentious subject in the sociology field, and there's a lot of competing explanations for this phenomenon. I reached out to an old colleague of mine, Prof. I.M. Bawling, author of Why Am I Crying? Tears, Wails, and Bellyaching Explained, and he was absolutely stumped. After a heated debate that ultimately resulted in the destruction of our lifelong friendship and revoking my invitation to Christmas dinner, the conclusion we've come to is:

It's a joke.