There is a theme that love Involves pain through the story. Cassie's first love, Connor, is killed by his own father along with the rest of Connor's family. Cassie is the one who finds them, and she holds Connor while he dies. She grieves for him for the rest of her life.
She loves her mother, but her mother is addicted to alcohol and thus is distant and preoccupied. Even her friend Ophelia, whom Cassie loves, is focused on her own gains and uses Cassie and Alex. Cassie has very few other close relationships in her life until she meets Alex. Her relationship with him is fairy-tale happy and they are a perfect match, except that he injures her regularly with his fists and feet.
Even giving birth to her baby, Cassie is gripped with unrelenting pain, but it is associated with the deep love she feels for him after he is born.