This is a difficult thing to analyze. This is the only explanation I can come up with:
In the beginning, the guy does things that initially attract the woman to him. He could be using social proof, being a challenge, myserious, C&F and even one of these may be combined with looks. This could make the female attracted to them. (I know you don't need looks to land a woman, but it's another element that will contribute to landing one)
I'm guessing that these friends of yours are hovering somewhere around 20 years old. Inexperience makes a first or second LTR interesting and exciting. When a girl is in her first or second LTR, she is still learning. Most likely she is going to end up with gift, flowers, cards, etc in these relationships. The attention she gets brings temporary happiness and feelings of "love". She'll be able to thrive on these temporary "love feelings" for a while.
Society also places a big part in this. The ideal relationship is portrayed as "the guy buys her gifts, ejaculates his feelings into her ear, and she feels love". Women know this is supposed to be a LTR full of love, but they don't constantly feel it.
When things get continually predictable, the relationship gets boring. For example, everytime they argue, he shows up with flowers. She can get flowers anytime she argues with him. She may also start an arguement deliberately just to test this out. She no longer gets that feeling of "love" when she recieves gifts, and she starts to pull away. The guy ends up spoiling her even more with gifts and favors, but they have no effect. She loses all respect for him.
She eventually breaks up with him, telling him one of the classic lines such as "I need some time alone", "It's not you, it's me", "I need to do things on my own", etc, etc.
After the female has had one or two LTRs, she gets turned off by "nice guys". She wants to feel some excitement, therefore she goes after the jerks. They're the only ones who can give her excitement.
