Why was Gatsby's dream a colossal illusion?

In F. Scott Fitzgerald's The Great Gatsby, Jay Gatsby was obsessed with Daisy Buchanan. But he was not in love with the real Daisy, but with an idealized, perfect woman who did not really exist. This is why his dream was an illusion--he spent his entire life to obtain someone who was not really as he imagined her.

Daisy Buchanan was a shallow, vain, self-centered woman who wanted a life of ease and luxury....

In F. Scott Fitzgerald's The Great Gatsby, Jay Gatsby was obsessed with Daisy Buchanan. But he was not in love with the real Daisy, but with an idealized, perfect woman who did not really exist. This is why his dream was an illusion--he spent his entire life to obtain someone who was not really as he imagined her.

Daisy Buchanan was a shallow, vain, self-centered woman who wanted a life of ease and luxury. She had been involved with Gatsby when they were both young, and he was poor. They lost touch and Daisy married Tom Buchanan, a very wealthy man. Gatsby was convinced that if only he had enough money, Daisy would leave Tom (and their daughter) and run away with him. Gatsby does indeed become fabulously wealthy, using various illegal means. He reinvents himself to become the mysterious, wealthy, society party-giver; he even changes his name from a more ethnic-sounding name (Jimmy Gatz) to one that is more socially acceptable. He does win Daisy back, but cannot see that she is not a person to be idealized. She is an indifferent mother to her daughter, stays with a husband she knows is cheating on her because of his money, and lets Gatsby take the blame for her hit-and-run murder of Myrtle Wilson. Gatsby still idealizes Daisy, even as she plans to leave with Tom and he, Gatsby, is shot to death by Myrtle's husband.