It's as useful or useless a question as to ask how all men or all women think. There is no one hard and fast answer.
Something to consider when pondering teenage girls and why they do the things that they do, and frankly, I think this goes for teenage boys as well, is insecurity.
Think about it, you have girls who are morphing into strange bodies, against their control, and these bodies are now the basis by which a lot of society is going to value their worth for the rest of their lives, whether it's if they have a pretty face or if they have bangin' bods, or if their skin is clear one day, or whether or not their hair is attractive omg on and on ad nauseam.
With teens, suddenly comes this idea of social status and fitting in. Along with this comes the idea of competition. Now all the other girls are competing for that same small pool of "acceptable" boys. Who decides who is "acceptable?" Well, society at large, and social conventions and ideals do change constantly, although the confident members are always the ones that stand out from the crowd.
Regarding the question of fitting in, in some cases, whether or not you have a good-looking or popular boyfriend hanging off your arm is the difference between whether you're in or out of certain social circles. Just being good at something or smart isn't going to cut it in the greater social pecking order.
Also, if you go too far in one direction or another, it can still hurt you. An example: my BFF in high school had a very curvaceous figure at a young age. Seriously, ALL of the guys at our school secretly or not-so-secretly wanted to screw her. This made the majority of the girls jealous, thinking that there was some other girl in competition for the hottest/most desirable guys. Consequently, they bad-mouthed her every day of school for 4 years, calling her a ****, defacing her car with some pretty awful graffiti. The kicker is that she never had a boyfriend and was a virgin until she went off to college. It was foul, what those ******* did, and it was all out of their own insecurity. They were insecure that the boys thought my friend was hotter.
For girls as well as boys, it totally sucks, being wrapped up in other people's validation of your worth. I was a late bloomer lol, and I remember going from 5th and 6th grade, pretty happy, outgoing and well adjusted, and then being slammed into the social ****-fest of later middle and high school, suddenly feeling as though I was worthless because a select small group of girls looked down their noses at me because I didn't have the same designer clothes as they had, and because the boys checked over my flat-chested figure and decided that I wasn't worth the time of day. These were the same boys and girls with whom I'd played since kindergarten.
Now, I understand that boys have it similarly. This is not some stupid pissing contest of who has it worse, men or women. Anytime any of us are judged and valued by shallow criteria, neglecting the true worth of the individual inside, we all suffer. Alas, it seems that it is a stinking rite of passage that we have to slog through on our way to adulthood. Mind you, some adults never outgrow this petty mindset. It's on my mind a lot because I have daughters who are embarking on this age. The agita, oh, the agita that is immediately ahead of me. Yikes.
So, to recap:
insecurity