If we're talking about Dazai ships, a huge part of it has to be the incredible character design Bungo Stray Dogs gave him. He's this walking contradiction—a genius-level strategist wrapped in bandages and suicidal ideation, cloaked in a flippant, theatrical persona. That complexity is pure catnip for fic writers. You can drop him into any dynamic and it's instantly charged. The 'canonically lonely' backstory, the hinted-at past traumas with Mori and the Port Mafia, the strange, almost parasitic friendship with Oda... it all creates these vacuums that fandom desperately wants to fill with connection, often of the romantic or obsessive variety. That craving for someone to truly 'see' him, beyond the performance, fuels so many pairings.
I see a lot of Dazai/Chuuya, obviously. Their history has that classic 'enemies-to-something' arc built right in, complete with a shared, powerful past and a present full of barbed insults and unspoken understanding. But I've also fallen deep into some quieter, more introspective Dazai/Atsushi fics that explore a different kind of salvation, or even speculative stuff with Fyodor that leans into the dark mirror trope. The beauty is, his character supports so many interpretations—tragic martyr, manipulative mastermind, secretly soft—that you can find a ship flavor for every mood.