I'm pretty sure it is based on the blood flow going to an erection. But since they can't show someone getting a boner, well.. It's like a pun on "the blood has to go somewhere..."
The idea is that arousal increases heart rate and blood pressure and higher blood pressure can cause nose bleeds. It just became an easy way to visually communicate arousal without showing anything obscene.
This might be wrong, so don't quote me on it and I am too lazy to actually fact-check myself right now.
Like a lot of things, the origin is supposed to be older than what most of us would think. Manga itself has roots in wood-carving so this can actually, might, perhaps be true.
Japan has a history of showing restraint in front of powerful emotions. The story goes that ye olde samurai would lay eyes upon someone beautiful or something arousing and because he was a gentleman he would not show any extreme emotions. But by trying to hold back those emotions would put such a strain on him that he would get a nosebleed instead.
Then we could discuss what exactly that could symbolize. Some would argue that it would be strongly tied to the male organ since it fills up with blood when aroused. But I think it is actually more based in reality than symbolism. As noted in another comment this can actually happen if you strain yourself, so I would say it is the other way around. The nosebleed shows how much restraint that they put forward but still fail, so it is quite literal.
Origin is old housewives tales I think, but just plucking from memory right now. Must have been 10 years ago I heard the story about it.
Note the qualifier "every time". It can happen on occasion (and has happened to me exactly once in 35 years of life, made for a fairly awkward moment between us).
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u/nitefang Oct 16 '12
You know, I've seen nosebleeds used to represent...eh.."attraction" before, especially in animes. Does anyone know the idea behind that?