@kinziebrooke I feel like I need a different account to answer this question, but I have too many kinks to be ashamed of it at this point, so idgaf
I'm guilty of being dragged into this kink accidentally, by jokingly reading a fic with it, thinking "hahaha this will be so funny", and then the fic was actually really good and well-written, and it wasn't so funny anymore.
I can't speak for everyone, but for me personally there were multiple aspects that played into it. I think the main one though, was letting go? Idk how to explain it well because I SUCK at writing, but like- y'know when you really have to go? But you can't, for whatever reason, and it's really hard to hold it. And then when you are able to let go, it just feels really nice?
...okay, I give up on explaining this in a sensitive way. Imagine holding in your piss for hours on end, and instead of going to the bathroom to let go, you go on your partner. Not only is it embarrassing (which is another kink that plays into it), but it's actually still really hard to let go. Whereas when you get to a bathroom after holding it for hours on end, it's relieving, your partner telling you to just let go while you're sitting in their lap isn't the same. It's fighting against natural instinct, and even though holding it is really difficult, you actually have to fight to let go, because all of your instincts are screaming at you to hold it. But being able to let go (and get relief) with you partner (plus the fact that they're turned on by it), is basically it.
Sorry if this grossed you out, but I did my best-