How is it a ton of fun? To me it seems to be one of the most boring things I could ever conceive. I am sure it's fun to people who do it however, and I am trying to understand why. Who knows, maybe once I understand it I will want to do it myself. I don't think this is an unreasonable question.
I'm having a lot of fun grinding mirrors for hours of a time. I am sure most people would not consider that to be fun, but boring menial work, but I am having lots of fun. I won't try to sell this activity to anyone else "because it's fun" as I am sure they just won't get it since they aren't into it already. It requires more careful explanation.
At a typical 'Ballroom beginners class', there will be no-one there who can already dance (i.e. if they were already great then they'd be in the next class up, etc). But also most people will have two left feet : Happy klutzes all around, without needing the excuse of alcohol to feel liberated. And there's always something to talk about... People are there just to have some fun, but don't feel the need to go 'CRAZY'. It's a nice atmosphere. Perhaps go for an icecream afterwards :-)
I presume you have a certain mental image of "ballroom dancing" that might not be entirely accurate, or at least is needlessly narrow. Many social partner dances put a heavy emphasis on self-expression, musical intepretation, improvisation, and partner communication. It might not be for everyone but it certainly can't be said to be boring.
The class may or may not be fun, but the extra skill can be. It's a physically intimate exchange between two people (in partner dances like ballroom emphasizes). If you enjoy music, it gives you another avenue for enjoyment and participation (actively responding to what you hear, as the lead choosing the right direction and movements for you and your partner, as the follow choosing the right flourish to add to a movement). Socializing is generally enjoyable for human beings, even introverts at least for a period of time, and this is one way to socialize. You also learn to let go which can allow you to experience more joy/fun than being overly anxious and rigid.
It's not a rigid thing. There are certain movements that everyone learns to start with, the basic waltz and foxtrot and such. Then you learn to combine different movements but still staying within the confines of the waltz. You learn how to add flair to it. It's like musical improvisation if you've ever played an instrument or sung, but physical, with your whole body. There are rules and constraints (really, conventions and guidelines). The restrictions impose challenges that can be fun to work within and around (a typical waltz is a 3/4 time piece, the dance is often done in a circle around the room, but what twirls and such you add are yours to choose and at the right time). Every dance will be different and unique.
I have to ask, do you actually have fun grinding mirrors for hours? Like, you actually experience joy and glee from the activity proper? Or is it the end product you enjoy or the challenges of finding the right way to achieve the qualities/properties you desire from a mirror with the tools and techniques at hand? Or is it, like running for me, meditative in its repetition, but still novel enough at each turn to not be mind numbing?