because of the foolish things that people do to make other people so annoyed that leads them to telling other people about how annoyed they are to a certain person and it spreads like gossip also..
When they initially "feel" their "crush" they don't know that this person does not like them - then when they find out they still have hopes things may change.
Usually because they dont like you, although they pretend ALL the time. I get that all the time, and sometimes its because they think they are the coolest people in the school, and they're too cool...