Other people conjecture based on case studies and anecdotes. Because so many of the most popular YouTube videos are either funny or cute — involving babies or kittens — you commonly hear that humor or cuteness is a key ingredient for virality.
But these “theories” ignore the fact that many funny or cute videos never take off. Sure, some cat clips get millions of views, but those are the outliers, not the norm. Most get less than a few dozen.
You may as well observe that Bill Clinton, Bill Gates, and Bill Cosby are all famous and conclude that changing your name to Bill is the route to fame and fortune. Although the initial observation is correct, the conclusion is patently ludicrous. By merely looking at a handful of viral hits, people miss the fact that many of those features also exist in content that failed to attract any audience whatsoever. To fully understand what causes people to share things, you have to look at both successes and failures. And whether, more often than not, certain characteristics are linked to success.
Excerpt from: Contagious: Why Things Catch On by Jonah Berger