Anne Frank was trapped in very small spaces with her sister, parents, another family, and another man for around two years. She was thirteen years old, which means she was entering adolescence. This is a time when most children would experiment with makeup and talk with their girlfriends about crushes. Anne was instead stuck in a tiny space with no privacy. She sought relief in what she had, and what she had was a little autograph book she had gotten for her birthday. Here, writing in the form of letters to an unknown friend, she could admit her secret thoughts, fears, hopes, and dreams. This was the only privacy, the only thing that belonged solely to her, that Anne had in her life for two years, as she grew from a little girl to a young woman.

What reason does Anne Frank give for starting a diary?  

The Diary of Anne Frank, also known as The Diary of a Young Girl, was an account written originally in Dutch of a family hiding from the Nazis during the German occupation of the Netherlands. Anne died in a concentration camp, but Anne's father, the only member of the family known to survive, had the diary published after the conclusion of the war.

Early in her entries, Anne explains why she started the diary:

Now I'm back to the point that prompted me to start a diary in the first place: I don't have a friend.

She goes on to explain that she has loving parents, a sister, relatives, and acquaintances, but she cannot communicate with these people about intimate things:

I can't bring myself to talk about anything but ordinary everyday things. We don't seem to be able to get any closer, and that's the problem. Maybe it's my fault that we don't confide in each other. In any case, that's just how things are, and unfortunately they're not liable to change. This is why I've started the diary.

Anne then explains that she wants the diary to be her friend, and that she's going to call her friend Kitty. Anne addresses many of her diary entries to her fictional friend Kitty. The inspiration for the character of Kitty was from the Joop ter Heul series of books for girls written by Cissy van Marxveldt. Anne read through this series both before and after she went into hiding with her family.

What reason does Anne Frank give for starting a diary?  

Before Anne goes into hiding, she starts her diary because she needs someone or something in which she can confide, as she does not feel that she can confide in other people. She writes: "I have never been able to confide in anyone, and I hope you will be a great source of comfort and support" (page numbers vary by edition). She received the diary for a birthday present when she turned 13 in June of 1942.