Why does Daisy tell Nick "the best thing a girl can be in this world" is a "beautiful little fool" in The Great Gatsby? Why does Nick feel as if Daisy's words are insincere?
That conversation takes place near the end of Chapter 1. Nick has just come over to Tom and Daisy's house after not seeing them for a few years. During the course of the dinner party, it becomes obvious there is a lot of tension in Tom and Daisy's marriage. Tom has a mistress, and she has called Tom at the house during dinner. Tom and Daisy snipe at each other, but try to pretend everything is fine to the guests.
Afterwards, Daisy and Nick go out on the front porch, and Daisy tries to tell Nick how miserable she is. With her words about girls being fools, she is implying all men are rats, women will always be cheated on, and it's best for a woman if she is fool enough not to realize what is happening.
Nick does not doubt Daisy is unhappy in her marriage. She then goes on to say,
You see I think everything's terrible anyhow. Everyone thinks so—the most advanced people. And I know. I've been everywhere and seen everything and done everything.
Nick recognizes this "everything is terrible" mentality as a pose Daisy puts on, or exaggerates, to underline how worldly and rich she is, "as if she had asserted her membership in a rather distinguished secret society to which she and Tom belonged."
In other words, Daisy is unhappy, but she is trying to elevate her garden-variety martial tension into a worldly cynicism she can be proud of.