Teens. The teen brain. Teen moods. Teen inconsistencies. Oh, and the "You don't understand..." line. The New Yorker ran a cartoon recently. Mom in teen girl's room, saying to her daughter: "It's you who don't understand me -- I've been fifteen, but you have never been forty-eight."
Aside from the laugh, here's my take-away. We all survive the teen years and most of us turn out all right. And teens simply cannot know (or appreciate) what they have not experienced or witnessed. They don't know the trouble we've seen. (And that is okay, too.)
I am an eclectic reader of periodicals. Around the time of that cartoon, I also read a well-done piece, "What's Really Going On Inside Your Teen's Head", in Parade.
Scientists are now looking at why teens seem to forget what they once knew; why they can be so bright and yet so clueless; and why all that sleep really is necessary. Read the article. It just might help.