Zachariah traps Sam and Dean in an office building where they don't remember each other to teach Dean about how he's meant to be a hunter? There is no way Dean would have done any of this without Sam by his side.
Zachariah takes Dean back in time to show that there's no other way for Lucifer to be defeated besides Michael? Dean realizes that he needs to reconnect with Sam because that's the only chance they have.
Zachariah kidnaps Sam and Adam and makes him feel like he has absolutely no other choice? One betrayed look from his brother and Dean immediately changes his mind.
He underestimated Sam and Dean's relationship every single step of the way. So did the other angels. And that's why the apocalypse failed.
















