A truly bizarre phenomenon.
Usually, things are either funny or not funny, and funny stays firmly in the eye of the beholder. There are some notable exceptions to this: Certain things are "comedy universals," such as pedestrians getting soaked by a bus driving through a puddle. Most things are funny in certain contexts, like farting or Manchester City FC. Some things just aren't funny, like winter. But the comedy of repetition is all of these at different times.
Let's work through an example. Imagine that a person makes a posting on a soccer website saying "Do you know how to date English porcelain? I really need to know." The regular users of the forum are likely to scratch their heads at the utter incongruity of this posting. A minute later, a response appears giving a perfectly sensible answer to this completely daft question, obviously cut 'n' pasted from another website. Now, either somebody's an extremely helpful individual and just wanted to oblige the poster, or it's been a set-up, with the initial poster supplying their own response, for purposes unclear. More head scratching ensues.
Now, imagine the above question-and-response is repeated every ten minutes for four hours. The other forum users are likely to become rather annoyed with this weirdo's "sense of humour", and much abuse will ensue, leading to people ignoring his posts.
But then something wonderful happens. The "porcelain man" changes tack, and starts fooling people into clicking on his posts by using the names of respected regulars, or interesting-sounding titles for his postings. Suddenly it's funny.
Look at the process: An honest answer to a sensible question is taken out of context and becomes absurd. Repitition of the posting out of context changes it from absurd to irritating. Eventually, though, familiarity changes it from irritating to funny.
Now, to keep the joke fresh, the poster introduces minor variations, for example "How to date English women", with a response in the same distinctive style and format of the original, but on a totally different subject. "I really need to know" becomes a forum catchphrase. Everyone hails the mad bloke as a comedy God and all-round great guy. Those in the know get one over on their uninitiated friends by repeating the phrase, and get knowing winks from those that are in on the joke. Before long, everybody's repeating this previously innocuous phrase, and for some all-too-brief period of time, the world's a happier place.
Then they get bored with it and it goes back to where it started.