Answer:
Brian was upset with having to bear the secret that his mother is violating her marital rites with another man.
Over the course of the story, we see Brian's thinking pattern evolve from one of a perturbed character who is vexed and frustrated by the situation in his family, to one that is tempered by the quiet environment of the Northern Woods in Canada. Here is gains mastery over self and the ability to help himself and control his emotions.
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