Boredom, soul searching, escapism. The search for their own identity and for meaning during the teenage years. Those are some of the recurring themes in this work. Also scary in how certain details and events that happen in this are surprisingly realistic. Wonder just how much of this is based off the Mangaka's own experiences...
It can get pretty deep at times, but then you'll find all of their struggles are actually meaningless and even naive when you look at the bigger picture. Teenagers looking for meaning where there's none and struggling with the lack of it. The need for being accepted by their peers, family struggles, there's a bit of everything here.