Why do people crush on people who do not like them back?

Primarily, it's because one people has waaay too much time on their peoples' hands, and becomes obsessed with another people due to some perceived shared interest. Perhaps it's a surreptitious wink, a nod of the head, or a T-shirt with the name of one peoples' "favorite band." The "crush" between People A and People B can take three courses:

• it can mercifully die out.
• one people won't let it go, and it never goes away, like syphilis, and becomes extremely embarrassing, also like the former.
• one of the people grows a pair and actually asks the other people out to coffee (note: this never happens, in practice).

The solution is to realize there are about 500 million other people in the world, easy, that would fit one peoples' target demographic. Or, people can just wait till high school is over, at which point the urge to have "crushes" on people should abate, unless people is some kind of sasquatch or sociopathic man-child.