What Keeps An Ad From Going Viral?

Have you got a favorite ad that's gone viral on youtube? Maybe one with roller skating babies or a soft drink? That's a marketer's dream these days – to have an ad so compelling that people watch it as entertainment, and tweet, blog and Facebook about it! Would you be surprised to hear that there's an entire branch of marketing research dedicated to studying what makes an ad go viral?

Per the Harvard Business Review  (which I peruse in support of the day job, not the romance writing LOL...