I don’t know if it’s a well-known fact —or if it’s a fact at all— that inside-jokes help a lot in build a relationship with anybody. Friends, colleagues, even people you like in a “couple” way, as far as I know, feel closer to you when you can establish a joke only the two —or few— of you understand (few, if you’re a group of people, in such a case people feel closer to the group, and not anyone specifically).
I don’t know the reason, and I don’t want to discuss it here. What I want to do instead is to relate this condition to a different angle. What if it doesn’t have to be a joke, but some sort of knowledge piece?
In the such a case, we’d be talking about two or more people who, based on some common experience (any kind, like visiting the zoo and watching a lion eat chocolate, for example), unconsciously create a piece of knowledge that only they have, and only they understand. Take that to a bigger scale and we could be talking about exclusive societies or organizations. Exclusive not in a way only a few can get in, but that just a few are in.
So, basically, knowing something others don’t makes you feel something. Knowing that thing, and knowing a person who knows that too, makes you feel closer to that person, as you think they feel the same way as you about it (perhaps?). From that, a feeling of exclusiveness creates around both of you, and will keep you a little closer.
In a way, doing things together creates those pieces of knowledge we treat as treasures. Sometimes I feel all the sharing in social media makes all that less personal, less valuable, and therefore, less likely to bring any group of people together.
Every piece of knowledge will transmit, if any, a different kind of feeling. It can be happiness as well as regret, or sadness. What I find interesting is that psychologist often say people need to share their thoughts in bad times, in order to “empty” their heads of bad thoughts, and therefore, feelings. What I really think it creates (if it actually helps), is a nexus to another person, with whom you’re sharing. You both know something no one else does, and that brings you together, which is a good thing in most cases. It’s like the sadness could be eclipsed by a semi-good feeling of getting closer to someone.
So, obviously, a more happy kind of shared knowledge could bring more happiness, as it adds up the “getting closer” thing with the actual happiness derived from the knowledge piece (a memory, a story, anything).
It’s said an inside-joke makes you and other person closer probably because a joke is always made to enjoy, an transmit good feelings. Sharing good feelings with somebody else is great, so that’s probably why an inside-joke makes a relationship work better.