I have an inherent issue with the trend of this gag in SPN. Because WHAT. GIRL. Has a one-night stand, and manages to remember everything BUT her bra?!
Those things are expensive! And it’s not easy to find good ones. Not to mention how completely awkward and uncomfortable it is it be in public without one (for most girls.) I mean, I can understand if she forgot her jacket, or her phone, or her panties even - since they might presumably be….less than ship-shape.
BUT WHAT GIRL LEAVES HER BRA BEHIND?!
(Also, apparently sexy girls only wear pink and black bras?)
Actually, I think it’s a very subtle and very clever illumination of Dean’s character — because you’re right, it isn’t funny. And it’s not supposed to be. Only Dean thinks it’s funny; the rest of us, including Sammy, think it’s obnoxious and actually kind of sad.
Particularly in the first five seasons, Dean is obsessed with proving how manly he is — which, due to how his father raised him, he associates with displays of machismo and hyper-masculinity. He derides any conversation that’s too emotional as “chick flick moments”, he obsesses over classic rock and fast cars (classic symbols of masculinity), and, most importantly, he chases lady-tail like it’s his job. It only makes sense that Dean would keep “mementos” of his one-night stands. They’re like trophies: tangible proof that he can point to and remind himself of just how masculine he really is.
All of this behavior conceals that Dean, as a character, actually follows a fairly feminine literary archetype, that of the Nurturer. His entire goal in life has always been to protect Sammy; he sublimates his own needs to help Sam become the man that Sam wants to be. (This culminates in “Swan Song”, when Dean was willing to sacrifice his own life just so Sammy wouldn’t have to “die alone”.) And unlike his brother, Dean is quick to cry, quick to anger, quick to reassure — always quick to display an emotional response rather than a rational one. For all his poor attempts at machismo, Dean actually displays many stereotypically feminine behaviors.
Why he does comes back, of course, as all things do, to John Winchester. On the one hand, John — a former marine forever stuck in 1979 — raised Dean in the hyper-machismic world of Hunting, and taught his son that a man never expressed his emotions or cared too deeply about anything. But on the other hand, John continuously reinforced Dean’s love for his brother and his nurturing characteristics until they became like instincts. In attempting to make himself a soldier, John actually inadvertently made Dean a mother figure too.
Thus, Dean is a man at war with himself, unable to reconcile what he’s always been told a man should be with what comes naturally to him. When you add his feelings for Castiel into the mix (which, no matter how you slice them, would be something John Winchester would definitely disapprove of), you get Dean’s major conflict of Supernatural’s later seasons: What exactly does it mean to be a man? (And as much as I love Sam Winchester, I find this conflict far more interesting and relevant than Sam’s, which is: What exactly does it mean to become a hero?)
When it comes down to it, Team Free Will is more than just a theological exercise. It’s Sam and Dean (and Cas, too) shrugging off of all the things they were taught, and learning that a man is not his past but his choices. That’s why Dean gives up John’s coat at the end of Season 5; why he forgives his own father and in turn becomes a father himself. It’s because Dean learns that a man is not who he is told to be, but who he decides to become.