I was once a depressed adolescent girl too, and for me it wasn't any scary bodily changes ("Are You There God it's Me Margaret" by Judy Bloom got me hyped) that did it but more the change in how my (female) class mates reacted to me... from 11- 14 was when cliques formed, along with the hierarchy of popular and unpopular, and it seems that many girls (such as I) had problems dealing with the new set of rules. And those youngster who don't conform to the new, quiet particular 13 year old version of social acceptability pay with ostracization.

It is very stressful to have your reputation or “social credibility” constantly at stake (or at least to think you do), and the easy relationships once enjoyed between friends suddenly warp as both children respond differently to hormonal/social changes... During adolescence I was surrounded by a fine amount of people, but still ended up feeling lonely because everyone (me too) was pretending to fit into a pop culture based off Cosmo Girl magazine, which at 11-14 I don't think anyone genuinely related to, so I just assumed that I was "different" and no one would understand me and stopped trying to honestly communicate. It was quiet melodramatic.