This is called using second person point of view. There are many reasons for this depending upon your interpretation of the text, but just like in other parts of The Things They Carried, "On the Rainy River" is intended to pull readers in to the process of the American draft and the Vietnam War. O'Brien is inviting us into the mind of the protagonist when he runs away to a resort for a few days, and wants us to imagine ourselves as a person who may have been drafted, or at the very least, who may have been required to do something scary and opposed to our own values, like war was for him.
