Because when girls do it, it's hot, and when guys do it, it's gay. It's a well known double standard.
To be honest, I only give my underwear to my friends in extreme situations... :|
And the reason we go to the bathroom together is because...well, Hermione got attacked by a troll, Ginny got abducted by a strange man, Katie got cursed, and Myrtle got killed. I think that's reason enough...there's safety in numbers!
Dat J.K. is trying to tell us something! I just can't figure out what.. Oh btw, 'safety in numbers' is an adage disproven already. Just look at the 'bystander effect', no safety there nuh-uh!
As for my experience of this: girls do seem a bit more comfortable in sharing than guys, even here in Sweden, most probably because of what we perceive as acceptable, what of this we implement and so on. Though I can not really say I've personally seen it to the extremes some here describe. I know how some share clothing, though never underwear unless the situation calls for it(emergencies etc.), go to the toilet together and do whoever-knows-what.
For me, as a guy, I don't see a problem in sharing as long as consideration for privacy is there as well. I don't mind lending clothes to someone, though I probably wouldn't do it to the same extent girls do it(those I know of), sharing a bed... If it's big and there ain't no hugging action, no probs really. As for food, I snatch some at times, because I can and they're not paying attention, deal with it.
Oh, and as to why this kind of behaviour came to be.. All is relative!
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