Why is this pairing a thing?

Very well, for my very first post, i want to ask you guys something, i´ve seen a lot of pairings(ships, whoever prefers it), but i cant help but notice that some of them have no reason to be, i dont mean the crackships such as Soarin and Braeburn, i mean like Twilestia, Twixie, Shimmerlight, Caramel and Mac, Shining and Sombra, Chrysalis and Sombra.

I get that people like to have fun with characters, but some(mainly the first ones) have some big fan back up, hell there is some sort of battle between mane 6 pairings, i could be exaggerating on some post and not getting this friendly jokely game.

Now before going on i wanna say this (cause you know, internet and stuff), i´m not against same sex couples, i noticed my examples way before writing this post, i support homosexual couples, i might not be gay, but i have friends and co-workes who are, i´m completely aware that i might not be able to understand some "why´s" and "because" due to being raised as a straight man.

That being said i want to propose the following, i want you, fellow bronys and pegasisters, to write down on the comments:
1- a pairing that you understand why exist, but doesn´t agree and why.
2-a pairing that you like, and why it could exist.
3-a pairing that you dont understand at all.
4-based on what you know, clarify why a pairing that other commenters dont understant, exist.