Liberalism is optimistic. It is a doctrine of progress and improvement. (Why do you think they call themselves “progressives”?)I can't speak for others who call themselves liberal, but I think my liberalism has generally stemmed from a deep well of pessimism. Just to pluck out three examples...
• I think that over time, an un- or under-regulated market will accrue all or most of the rewards to the people who already have the most resources, generally squeezing workers who actually do much of the wealth creation in that market.
• I think that, without a government to step in and safeguard everybody's rights, majorities will generally stomp on the neck of minorities—be they racial, religious, or sexual minorities.
• I think that when we go to war abroad, lots of people whom we never think about get killed. That it generally costs more and lasts longer than we're promised.
So I favor regulated markets, the rule of law, and a dovish foreign policy. Not because—as conservatives allege—I expect government to create some kind of heaven on earth. I know that's not possible. But I think government can curb our worst tendencies and mitigate their results. I don't expect heaven, but I do think we can—and should—work to stay out of hell.