Because: 
 - Flawed characters (both MC) 
 - Avoided cliches like: jock-taller-bigger-more-confident & geek-shorter-smarter-weaker-bottom. 
 - The theme (college break-up + mature make-up) is cleverly explored 
 - Two MCs POVs. 
 - The book is short but it focuses solely on the two heroes relationships, so their story seems longer than it is. 
 - The sex is hot, intense.