A lot of people who didn't go still had an experience with the Vietnam war. They had the draft back then. One day, you and a friend would be just walking down the street, and the next, one of you would be crawling through mud and muck in the jungle, or under arrest for protesting the war.
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https://www.psychologytoday.com/us/blog/the-secret-lives-of-introverts/201809/yes-you-do-become-more-introverted-with-age
(I'm sure there's some literary reference there that I'm missing, but this puzzle deserves investigation.)