Daisy is a true Southern belle of the old South, which meant that her upbringing instilled certain patterns of behavior in response to the expected roles she would have in life. She would have been raised with the understanding that her goal in life was to marry someone rich, and she would have been taught all the appropriate mannerisms and actions to support achieving that goal.
By the time of the story, Daisy's speech patterns and actions are completely natural - she has learned and internalized her lessons very well.
She...held my hand for a moment, looking up into my face, promising that there was no one in the world she so much wanted to see. That was a way she had...I've heard it said that Daisy's murmur was only to make people lean towrad her; and irrelevant criticism that made it no less charming.
Daisy is too superficial to be able to act in her relationships with others. Her voice and manners are completely natural and unthinking to her - she would not be capable of sounding or acting otherwise.
Perhaps the best evidence to show that Daisy is only acting a part comes at the end of the chapter, when she is alone with Nick and allows some of her true feelings to show. This is after she has argued with Tom regarding his affair as a result of the phone call that came during dinner. Daisy reveals how cynical she really is, and Nick is forced to see through her facade:
"You see I think everything's terrible anyhow," she went on in a convinced way. "Everybody thinks so—the most advanced people. And I KNOW. I've been everywhere and seen everything and done everything."
Her eyes flashed around her in a defiant way, rather like Tom's, and she laughed with thrilling scorn. "Sophisticated—God, I'm sophisticated!"
Instead of the delightful laughter that draws people in, Daisy has issued out a laugh of "thrilling scorn." When Nick hears this, he feels "as though the whole evening had been a trick of some sort." This theme of "acting a role" in society dominates the story, and every character will in some way behave as Daisy does.