Why does Anne want to keep a diary?
Anne wants to keep a diary because she doesn't have a real friend whom she can confide in. She feels that she and her existing friends are unable to get any closer, which leaves her feeling depressed and listless.
She has a strong need to get all kinds of things off her chest. She believes that "paper has more patience than people", making a diary a better listener than humans. Rather than just jotting down facts like most people, she wants the diary to be her friend.
Explanation
The textbook context clearly outlines Anne's reasons for starting a diary. Primarily, she lacks a true friend to share her innermost thoughts with, noting that she and her current acquaintances don't confide in each other. Secondly, she needs an outlet for her feelings and believes a diary will be more patient than people. She explicitly states her desire for the diary to be a friend, not just a record of facts.