Reality itself leads to the downfall of Gatsby's dream. What he dreamed of was ultimately unattainable.
Gatsby's dream was not merely to reunite with Daisy. He wanted to erase the last five years and go back to the moment in 1917 when they were in love with each other. He wanted to start over as if nothing had happened in the meantime.
However, as Nick tries to explain to him, that's not possible. There is no going back. Too much has happened in between. Daisy has married and had a child. She's built a life separate from Gatsby. Gatsby may refuse to hear this, but it is nevertheless true.
In the end, one person can't be forced to fulfill another's fantasies. Daisy can't become what Gatsby wants. As we watch him struggle with his dream slipping away (something he never accepts), we come to realize that it was the dream itself, more than Daisy, that kept Gatsby going.
On a more practical plot level, Tom interferes with the dream. He too has a stake in how events unfold. He too wants Daisy as his...
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