I think women are more obsessed with beauty because society has implemented us with such norms.
A lot of women also see beauty as power which in result causes them to want to go through plastic surgery and so forth. Whereas the male culture it's about putting the emphasis on becoming the high school jock with muscles as opposed to just looks.
It's kind of a stereotype to assume that all women are more obsessed with beauty than men. It may seem so for the most part due to social norms (as Emma said), which I think is due to our history of a male-dominated society and men's expectations of women. Yet today seems to be breaking all sorts of boundaries. I see many guys who are obsessed with how they look (I know some who spend longer working on their hair than I do in the mornings). Sure, I feel that the media emphasizes way too much on how a "beautiful" woman should look, but that's only because I'm part of the female group and can relate. I try to but can't really understand a male's position on this.
Of course, since we all have our individual opinions, why people in general are obsessed with beauty varies. Some people feel more confident when they feel beautiful, and some people just want to impress others. There are many reasons and many different interpretations of beauty, and it seems like people are just drawn to the idea of "beauty" because it is so different for everyone.
I think that society has a big impact on how we may see women as being more obsessed with beauty than men. Especially in today's culture, all around us we're always surrounded by things about beauty in magazines, on television and in advertisements. Many of these are geared towards women and it gives this idea that the goal is to fit in by trying to be beautiful. So in a way I do believe that women have been conditioned by society to feel this way.
Even though it may seem like women are obsessed with beauty, I'm sure many of us would say that men are also. Many guys have become obsessed with how they look in that they'd be willing to go to great lengths to get the perfect muscled body because this is what society has conditioned us into thinking. So even though we may often see women as being obsessed with beauty, I think there are an equal number of men who do care about their looks and the impression they leave.