Read the paper and I feel, while there's so much data and analysis, it seems overly academic. Academics need to understand that data often dulls the real, first hand sensory convictions and impulses that would explain things with much more brevity.
Trying to get a global aggrogate is also not very helpful, it's like trying to talk to different inhabitants of different planets on how they feel. Sure, there are some commonalities but without a cross-lingual, expert understanding of historical, cultural, economic and social differences, it's hard to be empathic to survey responses and tieing it to a global mileu.
It is exactly like the case of not seeing the forest for the trees. Kids today see the writing on the wall, hence they are depressed. They are disenfranchised by clowns in a circus with little prospects, other than to follow suit. It's going to be depressing to deal with the generation gap with the greatest lack of empathy and understanding of the world today.
It was a good paper to gloss over, but like many things that were already wrote here, it's not hard to find that a synthesis of all the issues and more, are the cumlitive answer as to the reasons why.