In Chapter 5, Gatsby's dream seems to be fulfilled, but what implies reality can't satisfy his dream?
Gatsby was in love with Daisy from the start. But in the five years he had been away from her, his estimation of her continued to increase. He had increasingly idealized her in his mind over this time. His dream of her became so great that it seemed impossible for her to live up to his expectations.
In fact, when they do meet again with Nick as a kind of chaperon, Gatsby is so nervous that he almost backs out of the meeting. There are any number of reasons Gatsby could be nervous. He might feel self-conscious, that Daisy won't have feelings for him anymore or that she won't be impressed by what he's accomplished. But he might also be worried that she won't live up to the dream of her that he has in his own mind.
The meeting happens and Gatsby is overcome with joy. But when Nick leaves them, he does notice a subtle indicator that the real Daisy has not lived up to Gatsby's dream:
As I went over to say goodbye I saw that 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.
Although he notices this doubt, Nick concludes that Gatsby either overlooks the doubt or he does not let the doubt affect his idealized dream. Nick adds, "No amount of fire or freshness can challenge what a man will store up in his ghostly heart." Even if the real Daisy has fallen short of his dream, Gatsby's romantic nature overrides it.