I think part of the reason for the stifness in Aragorn's case is that as a result of being raised in Rivendell he was probably raised in the manner of an elf child and so aquired a very elvish way of behaving. Had Elrond not extended his protect and had Aragon been raised among the rangers, he might have turned out as boisterous and lusty as Boromir. As for Faramir, I think the constant need for his father's approval is what led to his overly formal way, he was too afraid to trangress to be at ease.
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