Most japanese pwople don't get to experience their childhood fully and most of them never had experienced love in their teenage years. In America and Europe high school and college are mostly places to meet new friends, find a soulmate and get to some parties. While in Japan is not the same... Most of the students spend their days studying 24/7, not having enough time to meet friends. Some of them even work jobs while in high school and college, and this kills even more their freedom. They spend their childhood without being a kid and when they become adults they want to go back to these days. When they were young. When they could experience love and happiness. This is what makes them like high school girls or even younger ones. It's because it reminds them of the 'happy times'. It makes them feel comfort, as if they're reliving their younger days free. I can't blame them.