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It's all well and good to assume that Cas doesn't show Dean his true form because he thinks it'll scare him.
But I propose, Cas doesn't show Dean his true form because Dean is very proud of the fact that he's taller than Cas, (Half an inch still counts Sam!) and he regularly teases Cas about being short and Cas just doesn't want to take that away from Dean.
This eventually leads to a moment where Cas does show Dean his true form and Dean just stays silent during the interaction and a miscommunication trope happens because Cas thinks he's scared Dean :(
But in reality, Dean is flustered as hell and is trying to find the right words to tell Cas that he looks super cool/epic/hot in a totally #NoHomo bro type of way.