Reports say that he had no mental illnesses or anything he was just obsessed with her...
D is answer. (all of the above)
The message behind these words are that the boy should not think about what has happened now. He must move on and think about the future and how he will learn from this encounter.
Miller chose these words to be the last words of the show to explain (show) how even if things may go wrong and it seems impossible to change, you must think about the future and how this learning experience will benefit you in the future.
Hope this helps! (:
