This is a very interesting question. I think that her overall sense of positivism and courage underscores human beings' capacity to endure and suffer through challenging times with the hope of eventually triumphing over them. Her desire to maintain a sense of strength and positive energy in the most bleak of circumstances allows the reader to gain a level of respect for her which is almost unparalleled in other literature. The reader has little choice but to concede Anne's strength, and the millions of others to whose predicament she gives voice through her diary. It is not that she is excessively positive and resembles a partisan cheerleader. She is honest, sometimes brutally so. Yet, her toughness and unwillingness to lose her faith in the beautiful aspects of consciousness and her desire to resist the overwhelming social conditions of the time period become moments where we, as the readers, must ask if we would demonstrate the same level of courage in that predicament. In recognizing Anne's attributes, we hope to develop the same in ourselves. In this sense, the reader is affected by Anne in the same way as a mirror would hold an impact.
After the war, Anne Frank's father returned to the annex. He found Anne's diary and was surprised to find out that she had maintained such a positive attitude. He knew that she, in her death and through her diary, could show the world an individual who was innocent, kind, and has gone through such a rough experience, but still remained positive.
Anne Frank's story would have probably still influenced many readers even if she had not been so positive. However, her effort to continue to believe in humanity and remain positive appealed to many people through-out the world. I believe this inspired many people.
People recognized that if a child who was in her adolescence could remain positive during a harder ordeal than most people would ever endure, then they could learn to be positive during minor ordeals in life. Her positive attitude shamed many grouches and complainers. She brought a kind of humility to readers and helped people to love the child who ceased to exist simply because she was Jewish.