One way in which Gatsby failed to perceive reality is with respect to his beliefs concerning Daisy. He always held on to the belief that she really did love him, that she only rejected him because she was rich and he was poor, and that he could obtain her once he acquired enough wealth. He was able to obtain the wealth he needed, and he was even able to get her back into his life, but he was unfortunately not able to get her to love him in the same way he loved her. It becomes especially clear that Gatsby's perception of Daisy does not meet reality the day when Nick, Gatsby, Jordan Baker, and both Daisy and Tom Buchanan go into New York City to while away the hot day in a hotel room. While in the room, Tom begins to quiz Gatsby about Gatsby's mysterious past and exactly how he has earned his fortune. At first Gatsby tries to clear up any questions about his past, but when Tom only gets angrier, Gatsby exposes just exactly how he has misconstrued reality when he declares that Daisy loves him and has always loved him, not Tom. At first Daisy admits that she loves Gatsby, but then she breaks down saying that she did love her husband once and Gatsby along with him. This is Gatsby's first bitter realization of reality--that Daisy loved him as well as her husband. Moreover, he continues to have a great shock when Tom calls him a thief and a bootlegger, and Daisy's response is to shrink away from Gatsby, begging Tom, "PLEASE, Tom! I can't stand this any more" (Ch. 7). Nick's narration even asserts that it is at that moment when Daisy loses any courage to leave her husband for Gatsby:
Her frightened eyes told that whatever intentions, whatever courage she had had, were definitely gone. (Ch. 7).
Hence, one consequence Gatsby suffers from allowing himself to be deluded into believing an alternate reality is the heartache of having to come to grips with the bitter truth--Daisy never really loved him in the way he thought she had. A second consequence is that in allowing himself to believe Daisy loves him and associate himself with her, he also sets himself up for abandonment at the end of the story. The bitter reality is that Daisy is as equally immoral as Tom, so when Gatsby is killed by the end of the book, she performs the immoral action of completely abandoning Gatsby, even pretending he never existed.
Similarly to Gatsby, Robert Cohn is also disillusioned by love in The Sun Also Rises. When Cohn falls for Brett and they have a fling in San Sebastian, Cohn becomes under the illusion that Brett is also in love with him. However, Cohn's sense of reality, like Gatsby's is also shattered when he learns that Brett started an affair with Romero, the bullfighter in Pamplona. In actuality, Brett is incapable of truly loving any man due to her poor experiences with her husband; she is only capable of being sexually loose. Similarly to Gatsby, one consequence Cohn suffers from believing the false reality that he and Brett are in love is abandonment--by the end of the book, Brett has left Cohn for not only Romero but finally for Mike as well. In addition, Cohn's behavior towards his friends during his jealous rage is so terrible that he nearly loses his friends as a second consequence.