People are atomised to hell and back and the most accessible sense of belonging one can find is consumer identity. I don't know that young girls are in any way special in this. With boys it's primarily video games. Adults are not that much different.

Schools allow these so it's a status symbol of wealth. Yet another way to differentiate the "popular" (rich) kids from the "unpopular" (poor) kids

Capitalism inserts itself into the process of identity creation to create new feelings of lacking in its consumers. More “socialization” means more opportunities for “earned media” which makes people feel like they need to have The Thing.

Back in my day, if you didn't have a Nalgene you were a nobody

stickers on water bottles are fun. mine has stickers of mobile suits i got at a con. i have a sazabi, a zaku, and the turn a gundam