Thirteen Reasons Why is basically a book themed around bullying, suicide, and rumors. The book's theme can be interpreted in many ways. One theme that I came up with is "Little things can pile up." This theme can be seen from the way that Hannah Baker decided to kill herself because of thirteen different people, different reasons, not just one person, or one reason. She committed suicide because of the actions of thirteen people, but if it would have been just one or two, she would have been still alive.
Another theme that can found in the book is "Sometimes the best thing to do is to let things go", which is kind of the opposite of the last theme. Even though Hannah's small reasons piled up and it was depressing for her, maybe the best thing for her to do was to let them all go, or not hold on to them so much. She wouldn't have committed suicide if she had learned to forgive at forget the first people of the tapes.After a while, at the end of the tapes, Hannah practically dragged herself into bad situations because she felt like life wasn't worth living anymore because of the first five or six people. The last couple tapes were not the other person's fault anymore than it was of Hannah's fault for giving up. If she learned how to forget the events in the beginning tapes, she wouldn't have fallen into the situations later in the novel.
The bullying and rumors can also be transformed into a theme like "Don't start rumors because you don't know how much they will damage others." Most of the reasons why people are on the tape is because they had started a rumor about Hannah Baker or had talked about her in some way. This, I believe, was supposed to be the whole idea of the book, where a downward spiral of Hannah's life was started and accelerated by the rumors of her in her school. If it weren't for the first rumor of the book, then later events that happened later in the book wouldn't have happened to her.