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if dean said "i want you" cas would have heard "like an object." if dean said "i love you" cas would have heard "like a brother." so dean says "i need you" and he means "i need you like i need air." and we know this because dean has been trained to never need anything from anyone. he isn't supposed to need anyone. but cas does not know this and instead he hears "i need you like i need a gun."