Why does Anne Frank always feel positive about people?why does she feel that every one is good at heart?
Her optimism may have come in part from her age. It is difficult for young people to imagine their own deaths and hold on to the idea that nothing bad will happen to them. In the play based on Anne's diary, the last entry is
"I think the world may be going through a phase, the way I was with Mother. It’ll pass, maybe not for hundreds of years, but some day…. I still believe, in spite of everything, that people are really good at heart."
Whether Anne actually wrote those words or they were invented by the playwrights, they convey the sense that just as she has gone through the phase of always arguing with her mother, somehow the world was going through an awful phase of hatred. She just couldn't grasp the reality that people could be as evil as the Nazis turned out to be.
It's hard to answer your question because in order to understand, you have to almost put yourself in her place. That is something I would never want to do.