What reason does Anne give for starting a diary?

Anne starts a diary as a mean to share her feelings and experiences. She wants a friend that she can confide in completely.

June 12, 1942

I hope I will be able to confide everything to you, as I have never been able to confide in anyone, and I hope you will be a great source of comfort and support.

COMMENT ADDED BY ANNE ON SEPTEMBER 28, 1942: So far you truly have been a great source of comfort to me, and so has Kitty, whom I now write to regularly. This way of keeping a diary is much nicer, and now I can hardly wait for those moments when I'm able to write in you. Oh, I'm so glad I brought you along!