Family Bonds
The power of family bonds is the most prevalent theme in If I Stay. Mia's primary conflict—whether to stay alive or join her family in death—speaks to the unassailable connection she has to her parents and brother. Although they die in the early pages of the book, Mia's immediate family members live on throughout the novel in Mia's memories. But the pull of her memories only exacerbates the pain of being physically severed from her family. Mia's bond to her family is so strong that she decides it would be too difficult to live without and them. It is only once she recognizes the similarly powerful bonds of friendship and love that she believes it may be possible to live through the pain of her parents' and brother's deaths.
Friendship
The complicated but rewarding connections of friendship is another dominant theme in If I Stay. Woven into Mia's memories of her family are memories of how she first became friends with Kim. Mia's friendship with Kim was complicated by how they initially treated each other as enemies due to their similarities. This undercurrent of hostility is always present in their interactions, but their combative dynamic doesn't preclude intimacy and mutual understanding.
Life and Death
Because of the peculiar circumstance of her out-of-body experience, Mia has the rare opportunity to choose between life and death. In death, she would be united with parents, while in life, she would be forced to live with the unfathomable pain of their absence. Throughout the novel, Mia weighs each option, ultimately embracing life rather than submitting to death.
Free Will and Determinism
The tension between free will and determinism underlies much of Mia's conflict. The concept of free will dictates that people have control over the ways they behave and the decisions they make, while determinism understands free will to be an illusion and prefers to view human behaviors as the result of internal and external forces beyond one's control. Through her out-of-body-experience, Mia's fate split between free will and determinism: her physical body's fate is in the hands of doctors and nurses, while her detached mind believes itself capable of deciding whether to give in to death or struggle to survive. This theme becomes particularly relevant at the book's climactic ending: though Mia has decided to die, Adam puts headphones on her ears—an external influence that forces Mia's consciousness to reenter her body; it is difficult to say whether she chooses to live or the music involuntarily jolts her back into being.
Resilience
The question that underlies If I Stay is whether Mia is resilient enough to go on living without her closest family members. As Mia processes the horror and devastation of the car crash, combined with memories of how significance her parents and her brother were to her, she discovers that her past life is irretrievable. She believes the task of reinventing a new life in the aftermath of tragedy would be too emotionally taxing. However, once she considers the possibility of a bright future while simultaneously accepting the pain that lies ahead of her, Mia proves herself resilient enough to wake from her coma.
Love
The theme of love pervades If I Stay. Without the love of her closest family members, Mia can depend only on Adam's love. But Mia understands that there is a difference between the love strengthened by a lifetime of family bonds and the love she has cultivated with her boyfriend. Throughout the novel, Mia goes over her memories of Adam to evaluate whether his love is dependable and true. But while the awkward social differences between them lead Mia to doubt how Adam really feels about her, Adam's final gesture of love is profound enough to help her cling to life.
Social Belonging
Throughout the novel, Mia questions whether she belongs to any particular milieu, finding that her interests alienate her from her family, her boyfriend, and people generally. While her family and Adam are fans of rock music, Mia prefers classical, and she isn't sure why the genre compels her. Even among other cellists at her classical music camp, Mia worries she doesn't belong. Because she is less tenacious and competitive than the other musicians, she believes this must mean they are better than her. However, once she sees past surface-level differences and opens her perspective to the possibility that she is more like other people than she had suspected, Mia finds common ground with people in all parts of her life.