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Dean never thought he'd say "yes" to an angel. Having spent so much time being pressured to say yes to Michael by all of heaven but still holding on firmly to his "no". It was hard to phantom a situation where Dean would say yes to one.
Of course, things had always been different with Cas. There were very few things Dean wouldn't do for him.
So when Cas got down on one knee and asked for Dean's hand in marriage, saying "yes" to an angel was the easiest thing in the world. Saying yes to Cas was always easy.