As a female I have some insight into that. Basically it's two sides to the phenomena. One is the more "positive" I feel really sorry for her.... Let me say something nice so it'll make her happy for just a little bit....and everyone will see how nice I am.  The darker side is "I'm not feeling pretty...but I know I'm at least not as hideous as that obeast...so if I tell her she's pretty and other people do too, I can reassure myself that the bar is set super low to be pretty so I feel good about me.