In Chapter 5, Gatsby's dream seems to be fulfilled. What indications are there that reality cannot satisfy his dream?

In Chapter Five, Gatsby finally fulfills his dream of being reunited with Daisy, but it seems to be the beginning of the end for him. Fitzgerald uses the passage of time and the image of a broken clock to symbolize this end.

When Gatsby is with Nick and Daisy in Nick's living room, trying to be nonchalant, he accidentally nearly knocks over a clock which is sitting on the mantelpiece.

After Gatsby straightens the clock and sits down, he apologizes to Nick. Nick's response is, "It's an old clock," a response which he claims is idiotic, because the clock didn't sustain any damage. But then the text goes on to say, "I think we all believed for a moment that it had smashed in pieces on the floor." Since the near knocking-over of the clock occurs between Gatsby saying he and Daisy had met before, and saying it was five years ago, it releases tension between them. But the ominous statement that the three believed the clock had smashed is a little bit of symbolic foreshadowing. The clock is a symbol of time passing, and Daisy's next statement refers to the amount of time that has passed since she and Gatsby were last together. Gatsby has been waiting five years for this moment, but, like the clock, his hopes and dreams seem to lie smashed on the floor. It's as if the Daisy he has kept in his dreams and the Daisy he is sitting in front of now are completely different, so reality smashes his dream like the smashed clock of their collective imaginations.

A little bit further on, Gatsby panics, thinking he has made a mistake in asking Nick to bring Daisy over for a 'chance' meeting: "'This is a terrible mistake,' he said, shaking his head from side to side, 'a terrible, terrible mistake.'" Maybe Gatsby realizes for a few moments that he can't go back in time and re-live the moments he spent with Daisy five years previously. But Nick talks him into going back and talking to Daisy, telling Gatsby he's acting like a child.

The clock is mentioned again after Gatsby takes Nick and Daisy over to his house:

“It’s the funniest thing, old sport,” he said hilariously. “I can’t — When I try to ——”
He had passed visibly through two states and was entering upon a third. After his embarrassment and his unreasoning joy he was consumed with wonder at her presence. He had been full of the idea so long, dreamed it right through to the end, waited with his teeth set, so to speak, at an inconceivable pitch of intensity. Now, in the reaction, he was running down like an overwound clock.

Gatsby has dreamed of the moment he would be reunited with Daisy for so long that it has consumed him and been his reason for everything he does. Now that the moment is passing, Gatsby seems to be slowing down like a broken (unwound) clock. His life has been lived in the time leading up to his reunion with Daisy. Now that he has fulfilled his dream, his life seems to be over.