The Question and Answer section for The Diary of a Young Girl by Anne Frank is a great resource to ask questions, find answers, and discuss the novel.
Anne is inferring that her diary doesn't get impatient, angry, or judge her.... unlike the people in her life. She can tell her diary all of her secrets and be assured that her thoughts are approved of and accepted.
I take it you are referring to the stage play version. Unfortunately, I don't have access to the stage version of the book.
Are you referring to a particular diary entry or the diary in general?