>>90811363There are characters whose interactions you can find to be enjoyable in some way, so you want them in a position to interact more.
Sometimes you want a happy ending for characters you've grown attached to, and relationships are often seen as sort of hallmark of a satisfying life.
And maybe you just think interjecting that plotline would be amusing.
Perhaps you're a huge fucking loser, and project yourself onto a character, so you want them to be with your waifu, because it's as close as your lonely ass is ever going to come to human affection.
But as for cross-series crack ships? Shit man, your guess is as good as mine.