Thomas acts indignant because he kept asking too many questions. Like, who sent us here? Where are we? Why are we here? Why can't we escape this place?
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Thomas does not like feeling left out and wondering if something scary, terrifying, or fatal is waiting around the next corner. He does not understand the purpose of the maze and wants information. In a way he feels powerless; with the knowledge of the maze and its purpose and set up, he might have a way to conquer to the maze and deal with all of its frightening aspects.