A lot of books and movies use a tired theme that some people never grow up after high school or have a real life afterwords. It seems implausible that the stupid things people did as a teenager would forever put a stamp on their personality or that the relationships they had would significantly shape their long lives. The school years are really just a blip. The vast majority of people go on to meaningful and successful lives regardless of their teenage intellect or sports prowess or success with the opposite sex.
In the story, an old high school girlfriend, now much older, tells the protagonist that "he just doesn't get it, and never will". She explains nothing. This is like a mature person saying to another the infamous teenager line "whatever". Real people communicate better than that, at least most of the time.
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