Okay,prepare yourself for a wall of text. This is my opinion.
I always felt like it was a cultural thing. From middle school, and probably before that, I felt like I needed to worry about my appearance in relation to boys. Every magazine I read, TV show I watched, there's an emphasis on the beauty of a woman being extremely valuable. "lose x amount of weight!" "Keep the man" "wear/do this one thing to get him!" shit like that plastered all over the media I absorbed(i'm looking at you Seventeen magazine). It was really hard for me, personally, because I developed a very mature body at the age of 12-14. I just grew faster than everyone else, standing at 5'4' with curves and a C cup by seventh grade.
I got a lot of unwanted, sexual attention from men much older than me. Of course, I was 13 and had no idea what the hell I was doing. I had an idea, of course, but it was all confusing. In middle school I often was sent home or to the principals office for "inappropriate attire". The truth was, I wore the same damn thing as every other girl. I just had a more "womanly" looking body. The teachers and administration at my middle school made me feel like my body was dirty, like I was something to be ashamed of, and that I needed to cover myself for fear of distracting boys. I had a teacher tell me at the age of 14 that I needed to wear a shirt up to my collarbones, because apparently showing my "decolettage" is too much and I need to save it for my future husband. I wish I were fucking joking.
So basically, I grew up having this weird, twisted, complicated relationship with my developing body. I was told to both flaunt it and be a sexual object for men to admire and one day claim. I was also told that my body was a dirty, forbidden fruit to kept away from boys because they can't control themselves around a patch of thigh or shoulder. Now, these are two different views of the female body, but they result in one thing. Objectification. In both of these scenarios, I was told that my body was an object of male desire. For so long, I felt that I had no power over my body, that I was for male desire/control. That I was the object, and that males are the active subject.
I know now that this is sexist, heternormative bullshit, and that relationships between men and woman and sexuality are much more complicated than that. But I think that this objectification issue is one of the roots of a very complicated relationship between a woman and the way a woman views herself. I know that personally, this led to some self objectification. I ended up developing an eating disorder at an early age which still effects me today. I had a warped sense of my sexuality and really piss poor self esteem. I had a lot of gross, destructive relationships with older boys which really affected the way I thought of myself.
Anyways, I don't have a great conclusion for this, but that was my experience growing up as a female in the public school system who also consumed a lot of media. I certainly don't think it's this way everywhere and for every girl, and I definitely acknowledge that men and boys go through a lot of shit growing up too. This was just my experience, and I know it's the experience of a lot of other girls my age. :)