What is the theme in The Secret Diary of Laura Palmer by Jennifer Lynch?

The main concern of the book's central character, and therefore of the book itself, is the tension between being "good" and being "bad". Laura begins the book both believing and acting as though she is a "good" girl: good in terms of both behavior and how she is perceived by the community. As the result of her relationship with BOB, however, and as the result of other encounters (i.e., with Cousin Maddy), Laura discovers parts of herself that make her believe she is bad while, at the same time, she feels physically good. In other words, she believes her sexual, sensual side is bad, but experiences it as being good, with the tension between the two experiences defining and manifesting both the central conflict of the plot and the book's thematic explorations.