That pairing always felt like the logical extreme of their codependency to me. Everyone talks about the 'odd couple' thing, but it's way more twisted. It's not just a mismatched buddy comedy. Stewie literally tried to kill Brian in the early seasons, and Brian's tolerance for Stewie's sociopathy is its own kind of sickness. A romantic or sexual reading of that doesn't erase the animosity; it weaponizes it. The best fics I've seen use the ship to examine that thin line between profound, toxic intimacy and something resembling care. They're trapped with each other, the only two beings in that house who can have a real conversation, and that breeds a bizarre, exclusive bond. The pairing asks: what if that bond curdled into something possessive and romantic, but still kept all its vicious edges? It makes the humor darker, but also gives their quieter moments a strange, melancholic weight.
I once read a fic where Brian, in a rare moment of vulnerability, admitted he stays because Stewie is the only one who never expected him to be a 'good dog.' Stewie, in turn, saw Brian as his one constant experiment—a subject he never wanted to terminate. That messed me up more than any canonical episode. The pairing works because it doesn't sanitize them; it doubles down on their flaws and asks how love might exist within that framework, ugly and real.