what do you think anne start to write the dairy?

Did you mean, "Why do you think Anne started to write the dairy?

Anne was thrilled when she received the diary for her birthday. She didn't feel as if she had anyone she could completely trust or confide in (although she had numerous friendships), so the diary became the one friend (Kitty) she could say anything to without being misunderstood.

Anne got this diary on her 13th birthday. She was excited and thrilled because of this diary. So she named it kitty. She started writing dairy because she was feeling lonely although she had so many people to talk. It is said 'paper is more patient than man' this is applied in annes life.