Ok, so I know that we still know pretty much nothing about Alastor’s human life, and Rosie’s if she had one, but there’s so many reasons their dynamic is peak. Before I start I’ll once more reiterate that I’m not 100% certain of any of this but-

Alastor and Rosie seem to come from a similar time period. With Alastor living in the early 1900s and probs dying in 1920s or 30s in New Orleans. Rosie and her town seem to be based on ‘Hello, Dolly’ (ep 7 title even alludes to this) which was based in New York in 1890s. So not only would Alastor and Rosie likely never have the chance to meet in life but there’d also be a roughly 20 or more year difference between them. What I’m getting at is that while they’re so perfectly matched they’d never really be able to interact the way they do in hell if they were alive. Like imagine 20 yo Al dancing/gossiping/whatever with a 40-50 yo Rosie, while cute to us they’d probs be considered weird for their friendship, let alone a potential (lavender) marriage.

If Rosie was once a human then I’d imagine just like Alastor her cannibalism in hell would come from her interest in it from when she was alive. So that’d mean in life she’d likely also be a serial killer cannibal, just like Al. This creates a funny situation where they just so happen to be the exact same type of serial killer, meaning they lead very similar lifestyles and only they would actually understand this twisted part about each other.

Then in hell Rosie recreates and manages to maintain the early 1900s look and culture in her turf, but makes it cannibal friendly (lol). In this way kind of creating the perfect corner of hell for herself (and also Alastor).

What I’m getting at is that they’re qp soulmates who can meet and freely interact in an afterlife that’s kinda perfect for them (including the part where they’re allowed to be cruel). Like when ppl say you’ll meet your soulmate in heaven but those two are crazy so for them it’s meeting their perfect soulmate in hell.