It's in the early days of his humanity, back before he understands the concepts of "privacy" and "decency", that Cas decides that human clothes are just a little too constrictive.
Because, let's face it, going straight from powerful, giant, heavenly soldier to a puny, weak and painfully human body is quite the downgrade.
Cas can't help thinking that the whole concept of clothes in general is strange. Humans are cramped enough, living inside their tight skins, which wrinkle and crumple and stretch so easily, and Cas can't imagine why they'd want to make themselves more uncomfortable.
And it's not just that, no - that's just the start of it. His fingers fumble on the buttons of his shirt, and his belt is always too tight, and his pants bunch up in the most inconvenient of places, and don't even get him started on his scratchy socks.
After Cas fails to tie his tie for the seventh time that week, he decides the whole concept of clothes entirely is lost on him.
So he forgoes it all. He takes all his clothes and throws them in a heap in the corner of his bedroom in the bunker. It's all padding, really, or some strange attempt at fashion or individuality. Cas doesn't need to be any more individual - he's Cas.
Plus, he sees naked people on Dean's laptop all the time. They can't be wrong if Dean likes them, right?
Well, he's not wrong.
Part 1 of Two