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I couldn't forgive him or like him, but I saw that what he had done was, to him, entirely justified. It was all very careless and confused. They were careless people, Tom and Daisy – they smashed up things and creatures and then retreated back into their money or their vast carelessness, or whatever it was that kept them together, and let other people clean up the mess they had made […]. (9.145)
Nick may understand Tom, but he's not happy about it: he's dissatisfied with the way Tom and Daisy are dealing with this tragedy, and it's enough to send him scurrying back West in search of something else to be dissatisfied about.