Modern AU where they're coworkers/vague, passing acquaintances in very different departments at the same company. They know each other by name and stuff, but they don't really have occasion to work together. Then, while their respective teams are at the same corporate convention over one weekend, they take one of those in-depth personality tests and get matched as having perfectly compatible personalities. Later that night, they get drunk and have sex with each other.

Then they become friends, cuz why not.

But when they wake up some months later and realize they've insinuated themselves so thoroughly into each other's lives that they can't actually live without each other, (AND PINE), they have no idea what to do because they think the other person just thinks of them as an awesome friend.

(That personality test was 100% right.)