Some of these seem obvious, I am not a researcher or professor not am I involved with any sort of grant so I could be completely wrong but...
Blushing - When we become excited or anxious blood rushes faster to the face that it is pumped away so the increased oxygenated blood causes the reddish tint, seems like this one is more of a why does our heart rate increase when we are excited or anxious?
Pubic hair, we have hair around every part of the body that has more sensitive skin, head, genitals, inside of the nose, etc... Most of us don't have hair on the palms of our hands (you know who you are you unclean slashdotters) because that would interfere with tactile sensation needed for more dexterous tasks
Teenagers, apes don't have someone constantly trying to sell them a look or mood. Teenagers don't invent fashions and trends marketers do. Then a popular teenager decides that what the marketer said about it being cool must be true and their peers all wanting to be cool too follow. I am pretty sure my grandmother was not "emo". Again it seems like the question here is why do humans feel the need to follow the crowd.
Superstition - Somewhere along the way someone had a bad experience that they linked with another event (most likely coincidental) and they shared that information, and just as stories are retold superstition is passed along as well. There have also been some early studies showing that belief in superstitions may be a mild form of OCD.
Nose Picking, if your nose is clogged up you pick it to open nasal passages. Like any other behavior this can become habitual to the point of it happening unconsciously at inappropriate times.