What is the theme in Message in a Bottle by Nicholas Sparks?

Grief is a major theme in this book. As the novel begins, Theresa Osborne is running on the beach filled with thoughts of her son and her ex-husband. Although it has been four years since she learned of David's infidelity and three years since her divorce became final, Theresa is still grieving the death of her marriage. Theresa always believed that her husband was as devoted to her as she was to him and that they would be one of those couples who would be together for fifty years.