> The most plausible explanation for teenage unhappiness.

For any explanation to be plausible it has to take into consideration that America is not the world. We don't see the same sharp rises in depression amongs teens in europe, and guess what, europe has phones too, it has social media too. The explanation has to be rooted in something that's taking into account the differences between USA and the rest of the world. One thing that comes to mind is that children likley suffer trauma when they see students murder other students or shoot their teachers, or hear news about such events. But I don't quite think the size of the problem matches the size of depression statistics, even if rampant school shootings and gun violence are a uniquely american problem.