Right after Gatsby tells Daisy about the green light at the end of her dock, Nick notices a change in him. It is the green light that was a beacon to Gatsby for five years while he waited for this opportunity to reunite with her. Nick narrates, "Possibly it had occurred to him that the colossal significance of that light had now vanished forever." Nick follows this with, "His count of enchanted objects had diminished by one." Gatsby is certainly happy to be with her again, but the reality can't possibly live up to the perfect aura and idea of Daisy he had built up in his own mind. These enchanted objects and ideas like the green light suddenly have lost their luster. The reality of their reunion is great but the perfect vision he had imagined was too great to match in real life.
Later in the chapter, Nick notices a subtle indication of doubt in Gatsby. He notes, "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." For five years, Gatsby had idealized Daisy. All of his efforts went into building himself up as well. Gatsby's planning, effort, anticipation, and idealizations were all too great for Daisy to live up to his perfected dream. This novel describes the notion that the American Dream is an illusion. This parallels the "colossal vitality" of Gatsby's illusion.