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Why could "upbringing/parenting" be a theme in this story.
Because of the fact that Sam is very cautious, and the reason for that may be, that since he is black, he has been brought up be extra careful, because he is constantly at risk, because of his skin-colour.
The other men are not as cautious, because of them being white, and therefore they have not been brought up to be careful, because it has not been an issue for them.