How does the reader sympathize with Curley's wife when she opens up to Lennie in Of Mice and Men?
In this moment in the text, we become aware of the isolation, disappointment and naivety of Curley's wife. Where earlier in the story she appears to fit the derogatory description that accompanies her on the ranch (they call her a "tart"; a troublesome sexually charged person with more emotion than intellect), we see her in a new light when she speaks with Lennie in the barn.
Discovering that she dreamed of becoming an actress, we discover two things about Curley's wife. She is capable of girlish, naive dreaming and she is vulnerable. Her hopes were used against her as she sought a way to move away from home. (This is how she ended up with Curley.)
Her bitterness now makes sense in context. She has been deeply disappointed by the way her life has turned out. Instead of becoming a movie star and living in Hollywood, Curley's wife lives a friendless life on a ranch not very far from where she grew up.
She has been fooled. She has been disappointed. She has been isolated. Like Crooks, she tells Lennie that she has no one to talk to.
Both she and Crooks crave company and "someone to talk to."
The common misery of her life is easy to understand and, so, to sympathize with. This tendency toward sympathy is emphasized by the fact that Lennie has a friend. He has no trouble finding people to talk to on the ranch either. Though he has yet to achieve his dream of buying a ranch with George, we have encountered his joy of friendship already in the story. Lennie is fully aware that he and George do not suffer loneliness like Crooks and Curley's wife.
"But not us. And why. Because . . . because I got you to look after me, and you got me to look' after you, and that's why."