What reason does Anne give for starting a diary ?

Anne wants a confidant: a special friend to share her emotions with.

Emotions are precious and thus cannot be shared with everybody or anybody. We need somebody with whom we can share these. But then a true friend is as rare as the Philosopher's stone. Anne knows if she confides in a person, one or the other day, there are chances of betrayal. In her diary she finds a true friend and confidant. Besides, there was no fear of rebuke, mock or betrayal from the inanimate confidant.