Most of the time it's not because a girl just wants to be friends, it's because she knows you have feelings and gives just enough signs of interest in you to string you along between boyfriends because she likes the attention, and won't actually reject you.

People who get upset at "she just wants to be friends" are silly, rejection is a part of life, though I can understand if you can't/don't want to continue being friends (unrequited feelings suck).  People mostly get mad at being strung along, which everyone likes to pretend never happens.