If I were writing a paper about this, my thesis statement would be that women obsess about their weight because of the expectations placed on them by society.  Society's expectations give women a body image that is often out of touch with reality.  They have the idea that they must live up to the standard of various models (many of whose pictures are touched up anyway) who are not necessarily at healthy weights.

Our society expects women to be sexually attractive and judges them to some degree on that trait.  Standards of beauty in America are highly skewed towards a very thin look that is unattainable to most women.  Because they constantly feel that they are not living up to the standards of beauty that society sets, many women come to obsess about weight loss.