I think many players just want to be everyone's IC bestie, and assume everyone else approaches the game with that same mentality. Even if they behave ICly in ways that make being their bestie impossible unless you completely shut off your character's brain.
I've done the IC nemesis, and even just ICly frenemy, thing with players I trusted and were cool with it, and it's a lot of fun. It lets you play up different colors of your char's personality, so they aren't just blandly sociable in every scene.