Why do the Jews remain optimistic in the face of what Moshe tells them?

No one believed his story; some refused even to listen to him.

They refused to believe it was true because it was too frightening to comprehend. They also felt that bad things happen only to other people. At first they were anxious, but were soon optimistic again because the Germans were kind and not like they's been told. They continued to deny the reality of what was occurring around them.