One significant theme of the novel is the affect of isolation and loneliness on a person. This theme creates the a significant number of events in the novel. It is suggested that Ethan only proposed to Zeena because he was afraid to face a long winter alone in house after his mother died. Because this is a rather loveless marriage from the start, it doesn't have much reason to grow into anything more. When Ethan gets to know Mattie he sees how isolated he is even in his marriage and he starts to imagine how much better his life might be in a new place and with a new woman. Mattie too is an example of isolation. Her family is all dead and she arrives at the Frome's to help Zeena and because she has no where else to go. When Zeena sends her away at the end of the story she is terrified at the thought of being utterly alone in the world and suggests suicide as means to end it. She knows that she can never have Ethan, and that they can never be together any other way, so this seems like the only option.
Another significant theme that affects the story is society/social obligation. Ethan makes all of his choices in the novel based on what society tells him is the right and moral thing to do. He returns from school to care for his ailing mother; he marries Zeena because it is what young men should do; he stays with her because he took a vow; he doesn't take the money from the Hales because it is dishonest; he doesn't leave with Mattie because he would be leaving Zeena with no viable means to support herself; he cares for Mattie until the end because he is ultimately responsible for her injuries as a result of the sledding "accident." Ethan could have been a lot happier if he hadn't been so moral and conscious of society's expectations.