Question

why did Anne Frank write a diary ???

Answer 1

Anne appears to need is to vent her feelings. The first thing she writes it: “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.”
"Kitty"
While she’s in hiding, Anne decides to think of her diary as a friend, whom she names Kitty. It makes the writing easier. Besides, she doesn’t really have a real friend, ‘and that’s how the whole idea of keeping a diary started’, she admits. ‘All I think about when I‘m with friends is having a good time. 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.’

Answer 2

Anne wrote her diary for the same reasons any adolescent girl might write a diary. She wanted to chronicle her feelings and make sense of the world around her. Of course Anne was living in a pressure cooker. Hiding from Nazis with so many people in a small annex was not easy. Ann used her diary as a therapy to cope with not only teenaged angst but to keep perspective and her sanity in such a close environment. The original diary wasn't to just "Kitty" but to many pseudonyms. The diary became a sounding board for Anne and, eventually, a passionate piece of historic evidence for the world.