Nuclear armageddon almost happened multiple times in the past. To think it can’t happen is a necessary step in ensuring that it does.
Edit
Otherwise you end up with a nice contradiction: "how could this happen?! I was assuming it could happen, so it really shouldn't have!
Stanislav Petrov: https://www.youtube.com/watch?v=L7EmLf4Xlq0
Oleg Antonovich Gordievsky: https://www.youtube.com/watch?v=yprpoPc2g0k
Vasili Arkhipov: https://www.youtube.com/watch?v=iLokpu4ixQE
According to some comments "The Collapse Of Communism: The Untold Story" is a movie about Oleg Antonovich Gordievsky and there is a book "The Spy and the Traitor."
And Stanislav Petrov has a movie as well: https://www.youtube.com/watch?v=xTFUqnY3E3Q