Because you're not having enough interesting arguments now. Perhaps you have an instinct to want to grow and improve, but you're not able to easily access that in the here and now.

I sympathise. Sometimes it's not necessarily even about "winning" an argument, but just articulating your point well enough that things can move forwards. I find I am more likely to dwell on arguments where the other person was arguing against a point different than the one I was trying to make. In those cases, going over the argument in my head is usually me trying to think about how I could have better communicated my point in a manner that would allow the other person to engage with my actual points; perhaps then we could move forward productively and some change happens: either one of us causes the other to yield some ground, or I'm able to fortify my stance and come up with responses to new arguments. Either way, lack of closure sucks, especially when it feels stagnant.

Dwelling on past arguments isn't healthy, but I suspect you already know that, or you wouldn't have asked this question. Try not to beat yourself up about it too much though. Everyone ruminates sometimes, and trying to force yourself to not think of something is doomed to fail. It's more effective to find new things to focus on to help you to move forward, but that's an ongoing effort, and easier said than done.

To prepare for the next time. Whatever they said to use affected us and so the defense mechanism is to not let that happen again. Also regrets.

Well, the jerk store called, and they're running out of YOU!

Because, naturally, it works on improving itself. This is just normal behaviour. As far as Darwinism goes, you'd be pretty undesirable if you made it this far with a brain that shutdown every time it could idle.

Dictionaries call this "contemplating" and it's an incredibly powerful thing you ideally want your brain to do a lot of.

It has to be ego. I set my ego aside constantly and try to be open minded, and I am very open minded, but sometimes in privacy it's just so goddamn big for no fucking reason... Like when rehashing a 10-year-old argument in the shower.

I'd probably try to reflect onto how this topic or that person are relevant to you right now. There're random chances to just remember stuff, but if it drove you to post it, you think that's important, and this argument is probably unresolved or led to a less than enjoyable results.

stairwell thoughts

In my experience if there's a negative emotion (fear or shame usually) associated with a past situation which I don't want to feel I think about it again and again in seach of a perspective that basically makes me look better. It's futile. Best to just accept it as it happened, feel the feeling for a moment and then let it go.

https://en.wikipedia.org/wiki/Rumination_(psychology)