I need to find out the reason blowing your nose is seen rude in Japan. I keep searching and find things labeled, "Is blowing your nose in public bad in Japan". I know it is bad, I want to know why it is bad.
I guess you can speculate, and a Japanese person will probably come up with a justification.
But the most likely explanation is: It's is considered bad manners for no particular reason.
If a Japanese person asks you why Westerners consider it rude to slurp soup - which Japanese people regularly do - there wouldn't be a reason to that either. Or rather, for some unknown reason somebody, somewhen, decided that this was rude and started teaching that to the kids.
While people often have justifications for their customs ("it is more hygenic!"), those are often a product of their culture as well. For example, in some countries people may find it "hygenic" to be circumcised or to shave their armpits; while people from other cultures would not consider those practices to have any tangible benefits.
A quick answer is that "Because Japan is such a crowded culture. In Tokyo millions of folks are squashed together in close contact. Think about those crowded trains!
For this reason you have to be careful with hygeine.
A similar taboo in Japan is the "don't eat while walking" thing. You just don't eat or, say, drink a soda while walking along - it would be too easy to spill something on someone.
Here's a million-view youtube video by a Japan commentator. At one point the guy talks about exactly this .. "why?" these taboos such as the drinking one or the nose-blow one. unscientific yet insightful video
There's your homework! :)