Answer:
In The Great Gatsby, it was so awful for Gatsby to hear Daisy say that she loved them both because he had built up an idealized version of Daisy in his mind and believed that she loved him exclusively. Gatsby had been obsessed with Daisy for years and had built his entire life around the idea of winning her back. He had thrown extravagant parties and amassed a fortune in the hopes of impressing her and winning her love. When Daisy revealed that she loved both Gatsby and her husband Tom, it shattered Gatsby's illusion of her and made him realize that he could never have her all to himself. This realization was devastating for Gatsby, who had pinned all of his hopes and dreams on Daisy.
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