Yeah! I would like to state that I write all of my AUs with my partner @justsamwich! So I'm happy to answer asks here and you can also ask them questions over on their blog as well! Just wanted to make sure everyone knows it's not all me ^^

I will also put the names for the skeletons next to their respective AUs ^^

Undertale (Sans and Papyrus)

UnderFell (Grunge and Baroque)

UnderSwap (Ginger and Clove)

WesternTale (Doc and Ranger)

WesternFell (Rusty and Diablo)

WesternSwap (Ace and Rodeo)

MafiaTale (Smoke and Mac)

MafiaFell (Whiskey and Saint)

MafiaSwap (Romeo and Lucky)

Ooooh. Well, this is an interesting question~ here's what me and @justsamwich have for you, dear anon

The western sanses don't outrightly believe in it...

Doc wants to, but knows he would actively seek it out and he likes to keep himself more grounded. In a way, he does believe in it, but is in denial, as he would be sad to not experience it himself.

Rusty believes love takes a lot of work and that's part of the joy in having a relationship. He believes in attraction at first sight, but he feels that you need to really know someone before you can say you love someone.

Ace feels similarly to how Rusty does, believeing you need to know a person and on top of that, he's never experienced it himself so there's also no proof of it.

As for the papyri...

Ranger believes in it, but not for himself. He thinks it can happen to other people, but he's too focused on his own work to really want something like that for himself.

Diablo believes in it once it has happened to him and before that point, he thinks the idea of it is very sweet. He's never going to admit to it, but once it happens to him, well how could he not believe in it?