The passage is found in Chapter V after Gatsby and Daisy have been reunited, and he is taking her on a tour of his mansion. Nick watches Gatsby as Gatsby watches Daisy take in the opulence around her:
[Gatsby] hadn't once ceased looking at Daisy and I think he revalued everything in his house according to the measure of response it drew from her well-loved eyes. Sometimes, too, he stared around at his possessions in a dazed way as though in her actual and astounding presence none of it was any longer real.
With Daisy's presence, the dream Gatsby has pursued for so long finally has become reality, and he is having trouble separating the two. As their reunion continues, the contrast between Gatsby's dream and this new reality becomes more pronounced. Daisy, the real woman, seems diminished to him:
. . . the expression of bewilderment had come back into Gatsby's face, as though a faint doubt had occurred to him as to the quality of his present happiness. Almost five years! There must have been moments even that afternoon when Daisy tumbled short of his dreams--not through her own fault but because of the colossal vitality of his illusion . . . . No amount of fire or freshness can challenge what a man will store up in his ghostly heart.
While we chase our dreams and invest so much of ourselves into them, Fitzgerald implies, they take on a life of their own and can never live up to our expectations. For example, while Gatsby was longing for Daisy and dreaming of having her back, the green light on her dock assumed enormous emotional importance for him. After Daisy actually came back into his life, however, his relationship to the green light changed forever:
. . . the colossal significance of that light had now vanished forever. Compared to the great distance that had separated him from Daisy it had seemed very near to her, almost touching her. It had seemed as close as a star to the moon. Now it was again a green light on a dock. His count of enchanted objects had diminished by one.
Gatsby's dream seems to have "come true," but actually it has not. No reality could have matched the wonder of it.