They seem to believe in--or at least hope for--man's inherent goodness.
This is a good point.
I have noticed with self-described liberals I know (some members of my family), that they have a very hard time with the concept of evil. They seem to assume that when something evil happens, it is not because of the evil intent of a person, but some corruption of the system that creates the environment where someone is just about forced to do evil.
Of course the exception to this way of thinking are "conservatives" who are intelligent and in leadership. They are truly evil because they "hate" those they disagree with and are deceiving the unintelligent masses (the rest of conservative America).
It's a weird way to look at the world.
I don't know how many times I've had that quote from Anne Frank's diary used as a argument against evil:
“It's really a wonder that I haven't dropped all my ideals, because they seem so absurd and impossible to carry out. Yet I keep them, because in spite of everything, I still believe that people are really good at heart.”
It is a sweet and touching thought, but people who quote it seem to forget that shortly after she penned those words, her family was arrested and she ended up in a concentration camp where she died. Good but naive hopes and desires don't change reality.