Keith Burgess-Jackson, at his blog, suggests that conservatives are pessimistic about people while liberals are optimistic:
Conservatives are pessimists. They believe that human beings are essentially bad (evil, selfish, vain, power-hungry) and that the best we can hope for is that their worst impulses are constrained by religion, the family, community, and the state. Liberals are optimists. They believe that human beings are essentially good but are corrupted by society. If corruption is caused by society, then changing society will free humans to be good. Their innate goodness will shine forth like a diamond. This explains the liberal fervor to change (remake, engineer) society...Donald Luskin, however, disagrees. He thinks that liberals and conservatives are both pessimists:
I think liberalism regards individuals as weak and flawed, and seeks to use the power of government not to empower them to be virtuous, but to force them to be virtuous... That reining in of individual vice is the same thing that Keith says conservatives seek...I guess there is some logic to Luskin's view that libertarians are optimists about individuals, although one could also argue that in attempting to minimise the role of government, libertarians are also pessimistic about the capacity of the people in government to do good, which means that they are not wholly optimistic about people.
Now libertarians, on the other hand, are the real optimists about individual men. We seek to rein in the coercive power of collectives -- be it church, state, or corporation -- so that the largest possible number of transactions in the society are voluntary and individually determined. That's because we really believe in people -- we want them to make their own voluntary choices, right or wrong, win or lose. The only coercive role of the state is to protect individuals from force and fraud. Beyond that, we're optimists: you're on your own, and you'll do just fine.
Personally, I think that Burgess-Jackson's view is more consistent with reality: Liberals generally tend to see the good in people, at least as compared to conservatives. Which leads to what I think is a piece of irony.
In the United States, while the Democratic Party represents liberals and the Republican Party represents conservatives, the latter is also usually seen as representing libertarians, whose belief in individual liberty -- one would have thought -- should have aligned them more closely with liberals. It makes for some strange bedfellows -- to say nothing of contradictions -- in the American political scene.