You have to take a few things into consideration:
FIRST OF ALL... and Ive said it before... you cant expect EVERY HOT GIRL will work for you. A girl being hot isnt reason enough to marry her. Compatibility is reason #1. Some people get married and ralize YEARS later they just are not compatable in some situations. Compatibility issues cause sudden drops in IL
Second... missplaced expectations. I am sexy, but not sexually experienced. Once, I had seduced a girl with a BF of two years who had only slept with her BF before to take me to her dorm. She was all over me, ripping my clothes off and going down on me. I fumbled with the condom for 5 min, she looked at me really confused. After 5 min of less than mindblowing sex, she asked me to leave. Also, Im young. I go to bars and women meet me thinking Im "the one." They find out Im 18 and literally get depressed. First impressions lead to a lot of expectations, if you dont meet them, interest level drops.
Third... they find another guy. Self explanitory
Fourth... you dont escalate. Already elaborated on earlier in the thread.
Fifth... they get scared of their own attraction/attachement