I actually really enjoyed this movie. My life in no way mimiced the life of these girls, but it made an undeniable connection with my teenage years...though, much like others have posted, with my experiences/emotions ages 15-16.
I think it's rather silly that so many people are scoffing at the girl's drug use. She's a 13 year old freakin' girl. Did she really have to be doing heroin and selling herself on the street to evoke shock from you? It's not necessarily the drugs she was doing or the destructive lifestlye...but the shockingly speedy about-face that she made when she began the descent. She started off the film as a good student. Admittedly she had some previous issues dealing with her own life's unhappy circumstances, but she retained the innocent qualities of what you would expect from a 13 year old girl. She was wearing socks with yellow ducks on them and had a bed littered with stuffed animals. Her best friend was probably the same one she'd grown up with since childhood. Within a few days she made an insanely manic change, not one that developed over her teenage years (which is why I suspect most people connect with her at 15 or 16...because most people slowly progress through these changes).
As far as pity goes, yeah, I do feel pity for her. You don't have to have a broken, drug-addled, physically/sexually abusive family to find your own personal Hell. You can find evidence of this in any rehab clinic. There are people who have completely thrown away everything in life: woke up only with the hope of consuming more drugs, watched "friends" die, burned bridges with family and true friends, took their bodies to the brink of death and back...blah blah blah. And then you've got the guy who took his drinking a bit far, recognized the problem a month later after shaming friends and family...and to himself, his experience is just as gut-wrenching and shaming as the dude's who just woke up in a gutter.
Everything is relative to the individual's perspective when it comes to depression and shame over actions.