Why do girls make such a big deal out of prom?

Ok, so my senior year i had a girlfriend.I was supposed to go to prom with her, but my grandma wouldn't let her ride in the limo that she rented, because she didn't like her.So i asked this girl that was in my class to go with me,she agreed,If i remember correctly she bought her dress and everything,then my sister's friends cousin wanted to go to prom with me.So i told the girl in my class i couldn't go with her,she seemed really sad/mad at me.She's finally just starting to talk to me again.Why do girls make such big deal out of prom?