I see now there was a reason for it. He knew he had a big future in front of him.
At the end of the novel, Nick comments on Gatsby: “Gatsby believed in the green light, the orgastic future that year by year recedes before us” (180).
The future is splendid and enthralling, but it also always seems out of reach. However, Mr. Gatz has faith in the future just as much as Gatsby does. In a sense he is worshipping his son, admiringly explaining his running away as a teenager, because James/Jay has made a fortune and climbed up the social ladder. Gatsby justifies himself as far as Mr. Gatz is concerned because he attains something that his father and grandfather cannot achieve.
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