I shook hands with him; it seemed silly not to, for I felt suddenly as though I were talking to a child.
At the very end of the novel, Nick finally comes out and says explicitly what he thinks of Tom, and, by extrapolation, what he thinks about the upper class. He compares Tom to a “child” because he continues to behave immaturely despite being a fully grown man. Since he has always been able to hide behind his money, as Nick points out in the preceding passage, Tom has never learned to take responsibility for his actions, and that allows him to juxtapose his lack of moral code with his completely clear conscience.
Just like a kid throwing a tantrum, Tom believes he is being wronged whenever he suffers the consequences of his acts, regardless of how unethical or stupid that act may have been.
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