The story I'll tell now, I read long ago. It's disturbing and chauvinistic, and although it was presented as a real experiment where I read, I think it is just a made-up story. I will use it here, because it is a useful metaphor, more like a warning (to myself and any girls who might be reading this.)
Male rats and female rats were put in a box with an electric wire seperating them. After a while, a male rat attempted to jump to the other side, but was tangled in the wire and died. None of the rats moved for a while. Then, seeing that the male rats weren't doing anything, female rats started jumping over the wire one by one, falling dead one after another.
We girls are used to doing our best. We think that we can attain everything if we work hard at it. We believe in meritocracy. But relationships require mutual effort, and it won't work if it is only us who is trying to make a relationship work. Besides, if we change ourselves too much, we won't be the same person he loved in the first place. So doing your best isn't always working hard. Doing your best is realizing what works and what doesn't, putting effort in the right person, protecting and preserving yourself. (For more on this, read "He's Just not that Into You.")