Gender roles
Gender roles as they are traditionally understood play a central role in the story. To begin with, Mattie is only called upon to help her dad on the farm because her brother is shirking his responsibility as the only son to work on the farm. Since he won't do what he should, Mattie, a girl, is forced to do the work of a man. Then she gets the idea that she would help her dad better and further her own dreams if she could get a paying job in town, but this is not a traditional route for a young woman to take. When she helps Maddie deliver her baby, Mattie realizes that if she were to marry she would be called upon to endure that same painful ordeal of pregnancy and childbirth. In the early twentieth century, marriage without children was unheard of, so Mattie determines never to get married so that she doesn't have to be a mother.
Grief
When Mattie's mom dies, she asks Mattie to promise that she'll stay on the farm and help her dad. Mattie consents. But this means that Mattie cannot go to college, which is her personal dream. If she weren't consumed with grief after losing her mom, Mattie most likely would have the perspective to know that giving up on her dreams is not what her mother intended for her. This blindness as a result of grief also gets in the way of Mattie's relationships because the people around her who care about her can see that she's unhappy because she's holding herself duty-bound to this promise to stay on the farm.
Independence
Mattie is young, just growing in to her womanhood. By choosing to go work in Adirondacks, she is defying her father's expectations of her. She lives on her own, which is pretty unheard of for the early 1900s. Over the course of her job, she develops a friendship with the young couple who disappears. Because of her presence of mind and decisiveness, Mattie is able to help solve Grace's death and her husband's disappearance.
Obligation
Mattie makes her decisions largely based upon obligation. Whether to herself, her lovers, her dad, or her dead mother, she is always trying to fulfill an obligation to somebody. The most notable is her promise to her mom on her deathbed to stay and help her dad on the farm. Then, her dad gets the idea that Mattie will always stay and maybe even take the place of her disappointing brother. Unwilling to let down her parents, Mattie stays on the farm a lot longer than she maybe should have for her mental health. In her other relationships she again acts out of a sort of compulsion to please everyone. Her choices are based upon a moral imperative that she be the best she can to everybody because she has to do it.
