There's this idea among the folks studying oral traditions that some of the "formulas" used - little stock lines or half-lines of poetry the poet could summon up and insert entirely into a piece - carried with them resonances, deep and metaphorical, that made them linguistically rich, rather than empty repetition.
I'm hazarding a guess that in-jokes work in a similar way. They reinforce an idea of community (those who get the joke) and imply a history when invoked (all the other times the in-joke has come up). To those outside of the community, the insiders' enthusiasm for the in-joke seems incomprehensible; the outsider hears only some witless words, repeated ad nauseum.
MY HYPOTHESIS: LET ME SHOW YOU IT
You either just entirely understood what I'm talking about, or have no idea why I feel the need to scream stilted English in all caps.
Let me explain the all caps thing, there. It started, as far as I know, with this lolcat (captioned cat picture). But that's really the mid-point. To understand why the picture is (allegedly) funny, you have to know something about Pokemon and the people who play it. Pokemon is a collectible card game (and handheld video game) (plus cartoon/toy franchise); like baseball cards, you buy packs of cards in the hopes of getting the ones you want. It's aimed at kids in roughly the 8-12 year old zone. Have you ever met an 8-12 year old kid with a collection and a lot of knowledge about it? They want to share. They want you to know about their hobby, because it's awesome and they love it, and you will love it, too. YOU WILL LOVE IT TOO. They just have to tell you about it enough, and you will be so happy to know the joy of [their favorite thing].
I have been that kid, for the record. It's not uncommon; it's part of child development. So a good number of people have met a kid who wants to tell them all about [their favorite thing], here exemplified as Pokemon.
Which further draws on stereotypes about geeks and nerds and their inability to read social cues. Because not only are we talking about an enthusiastic kid here, the enthusiasm is for a bit of geek culture. And not a few adult geeks retain the desire to tell you all about [their favorite thing], whether or not you care. You may be this person; you may know this person. You recognize this person.
So the cat picture moves us to smile. And then it moves us to imitation. The "My [thing], let me show you it" phrase becomes popular - the kids today call that "going viral," yes? - and it becomes a point of reference. At worst, it is witless parroting. At best, the phrase encapsulates the consuming enthusiasm and delighted, past-all-reason desire to share that enthusiasm, while at the same time acknowledging that perhaps yes, this is on the crazy side. When I say "My hypothesis: Let me show you it," I am saying, "I have this idea and I'm really excited about it. I'm going to write a few hundred words on it, which might be way more than it deserves. I really hope you'll read it and like it, because I think it's a neat idea! But really, I'm talking about lolcats and oral tradition and I understand that's a little specific for 10:30am on a Monday morning. Or, really, anytime."
Take home lesson: This is why filks and pop culture references will never die in the SCA.