If I Stay by Gayle Forman is the story of Mia, a young girl, who is in a car accident with her parents and younger brother. Mia is then trapped between the afterlife and the real world. She watches in an out-of-body experience as her friends and loved ones come to spend time with her in the hospital, and she has to decide what she will do: stay and live or go on to the beyond.
In the book, Mia has to decide if she will stay and live her life without her parents or her brother (they all died in the accident) or go on now that her life has been drastically altered. She says:
If I stay. If I live. It's up to me. (Chapter 9)
She understands that she has a choice. A life without her family will be difficult, and she has to decide if she wants to come back to that type of struggle. The choice is a huge focus in the book. It takes up much of the central plot and it functions as the central conflict in the story. Every other theme is secondary to the choice, but all other thematic elements (love, family, identity, etc.) play a part in her decision.
Her decision to stay or go centers around what's left of her life. A life without her parents and little brother won't be free of pain, and it would be really easy to slip into death. Mia describes death like slipping into a warm bathtub, which sounds relatively easy compared to trying to live in the shadow of her immediate family's death.
Mia plays a flashback in her mind and remembers back to something Adam (her boyfriend) told her in the past:
The you who you are tonight is the same you I was in love with yesterday, the same you I'll be in love with tomorrow. I love that you're fragile and tough, quiet and kick-ass. (Chapter 9)
Adam is one of the key reasons she chooses to stay—precisely because she loves him. If he hadn't been in her life, and if he hadn't said that he would love her day after day, it is unlikely she would have chosen to stay. She lost so much, but having Adam is one of the few reasons she considers staying.
Another factor in her decision to stay is her friend Kim. Kim explains that she still has a family despite the death of her immediate family, saying:
You still have a family, she whispered. (Chapter 16)
Kim reminds Mia that there are many people—including other relatives, Adam, and her friends—who all care about Mia. They all want her to stay. For this reason, in addition to her plans for the future, she can pull herself back to the present and decide to stay.