I'm wondering why are teenagers so obsessed with depression and anxiety.etc.?

Obsessed?: It depends on what you mean. Dramas, angst, relationship issues are the domain of teens. They are prone towards hyperbole and exaggeration, sometimes due to the lack of perspective and time in life. Major depression and generalized anxiety are serious conditions and they are not the 'teenage dramas' at all. They are real and require assessment and treatment.

Agree w Dr. Vertino.: I also agree that one must separate the angst of teen existence from the conditions of depression ; anxiety. I very much doubt that teens that suffer from depression or anxiety desire it. I personally am not aware that teens in general tend to obsess about depression or anxiety (any more than adults).