The shift from first person to second emphasizes the confessional nature of the story, and assigns the narrator's feelings to the reader. In that sense, the use of second person serves to break down the distinction between writer and reader. Even though the narrator is recounting her specific experiences, we begin to understand and empathize with her situation.
This is all accomplished quite naturally from a stylistic point of view. Minot shifts back and forth from first to second person in a way that does not call attention to itself. Take, for example, the first occurrence of this shift, quite early on:
You’d go on walks to get off campus. It was raining like hell, my sweater as sopped as a wet sheep.
The first sentence, in the second person, makes an generalization: this is what you'd do, meaning what everybody would do, as if it were unavoidable. The "you" suggests that anyone, including you, reader, would do this. The next sentence is back in first person, and describes a specific detail about a particular walk, something that happened to the narrator. The juxtaposition of these two sentences underlines the specific nature of her experience while at the same time suggesting that experiences like this are universal.
Another function of the second person is to assign gender to the reader. In addressing the reader as "you," the story explicitly addresses a female reader. If you happen to be a male reader, this assignment can be challenging, but it makes males readers see things from a female point of view.
This is in keeping with the theme of the story, which is to provide some context for the narrator's attitude toward men. Even though the narrator may have had experiences that are different in their particulars from the reader's experience, the story serves to show the underlying feelings of sadness and emptiness that guided her.