It is your age bracket bud. Most very social girls in the early / mid 20s are not really looking for a serious man. They like playing their options, having fun, and using guys to help their social class. Now by the time they turn 28 then they change their tune. They start seeing their other girlfriends who were once as slutty as them now they are settling down, getting married, and having kids. So then the 28yo girl feels like the music is about to stop and she'll be left without a chair....then she is motivated for serious dating.
(Or maybe I just have a sordid past with the wrong kind of women)