I may be wrong on the following, it's just what I feel, basically it's gotta do with the mentality when one gets into a r/s.
a) Some girls are in a r/s coz they want attention, love, care from the bf. Meaning, bf gives more than gf then gf is in cloud nine. Bf gets busy from other committment, gf finds bf shortchanging her, comparing from previous 'honeymoon period' where bf gives all gf wants, thus the argument about not being sensitive and craps.

Everyone needs to recharge themselves, girls and guys alike. They recharged differently. Guys go back to their 'cave', girls sink to the 'well'. Guys want time off, so that he can come back stronger and better, girls sink so that they can rise to the occassion and be loving again.
b) Some are in love head over heels over this guy, (infatuation perhaps, I dont know..) it's a challenge to them if they can possess him, after the catch is in the net, it's no longer a thrill to have him. thus r/s cooled down, guy thinks gal is not interested anymore, which can be the fact. (it can happen on a guy to a gal as well)

dont get into a r/s just bcoz of superficial qualities one possess. If one can like you for looks, then surely there will be a better catch out there, one who possess better outlook than you. When one is madly in love, flaws r nuts to them. When one loses interests, the mere sight of him/her yawning can be detestful.
c) Some gals are 'post it pads' - ever sticky to their bfs. (Possessive? I am not sure abt it, but their beliefs will be that of, I want to be where he is seen, want to be around for him, etc etc.) Guys hang out with friends, friends tease him for having a superglue. If he dont turn up for their meetings, then he'll be accused of 'Got gf, dont want friends'. Now r/s becomes rocky bcoz gf now finds bf not spending time with her, which to the bf is he wants time off to mix with his friends to prevent his ego from being pricked/teasing/misunderstandings/etc etc etc.
d) There's this grp of other gals, who r in a r/s bcoz of the qualitative characteristics of their partner. Wants bf to set his priorities, feel/think for the bf. Doesnt need much of companion - independant type. Have the mindset that even though in the r/s, both are still individuals with goals in mind. Talks about spending quality time, not quantity time together. So even if bf doesnt give alot of attention on her, she knows she can trust that he'll be around for her when she needs it. Gives bf freedom/trust/respect/etc to do the things he wants to do.
Realise there's a trend on the behaviours of the gals, depending on their maturity level. Of coz, maturity level does not equate to age all the time.
Which grp of girls you have gotten yourself for, depends on what you can provide for them. Ultimately, no two peas are alike. You cannot find 2 people with the same characteristics/mentality.
I may be wrong on the above, just my 2 cents anyway. Take it with a pinch of salt yeah?
