In 'dear john novel', the theme might also revolve around self - discovery. As the story progresses, the characters could be on a journey of finding out who they really are. Their experiences in love, whether good or bad, can be a catalyst for this self - exploration. They might learn about their strengths, weaknesses, and what they truly want in life through their relationships depicted in the novel.
Well, I think the theme could be sacrifice too. The characters in 'dear john novel' may have to sacrifice their own desires or plans for the sake of love or other important things in their lives. For example, one character might sacrifice a good career opportunity to stay with the person they love. It shows how love can make people give up things they value for the sake of the relationship.