this is a great thread! ive definitely been through it with ships, but the main pairing of Stains is by faaaaarr my favourite, and came through super organically (i was writing a different au as writing practice and the MCs were roommates and well, they turned out to be perfect for each other). Casper's original romance is one that, since getting more realistic with my relationships, turned from being a 'guy forgets his troubles for his bf' ship, to realising that it'd actually just be an ... awful, awful relationship no matter that it sort of sticks, and so Jack's become almost an antagonist in the current novel.
Casper and Cain might still be 'good for each other and no one else', but they hit off each other SO WELL and they end up communicating too well when i write them to even write the 'miscommunication causes relationship issues' angle to stuff. I just can't even make it believable that they wouldn't sort stuff out before storming off and almost ruining things (hence the somewhat 'external' conflict to the romance in stains)