I agree with R. R. Reno’s assessment, presented in his review of The Uses of Pessimism: And the Danger of False Hope, by Roger Scruton (non-subscribers can try here):

Scruton observes that “the belief that human beings can either foresee the future or control it to their own advantage ought not to have survived an attentive reading of the Iliad, still less of the Old Testament.” But hope springs eternal. The successes of modern science provide one explanation, for they encourage what Scruton calls “the careless pursuit of mastery.” If we can control nuclear reactions, then why not the growth of cities or the education of children or the workings of a modern economy? We program computers, so it seems natural that we should treat social mores such as traditional forms of marriage and child rearing as silicon chips we can overwrite with new codes.

I disagree, however, with the underlying reasoning that appears to leave both reviewer and reviewed to that conclusion:

Without pessimism, we tend to become what Scruton calls “unscrupulous optimists,” those who “believe that the difficulties and disorders of humankind can be overcome by some large-scale adjustment.” Belief turns into action, and grand plans for social change demolish and destroy inherited ways of life to build such empires of hope as urban renewal, wars on poverty, and, of course, the mother of all hopes, the classless society. Modern societies are filled with witnesses to the failures of optimism, from the empty concrete plazas conceived by urban planners to the demoralized population of the former Soviet Union.

At a more basic level, I think this has it exactly backwards. The notion that “we have to do something” is more an expression of pessimism — as in, “without us, all is lost.” Yes, an unjustified optimism may come into play with the assessment of success’s probability, but that’s hardly the defining characteristic of meddlers. One needn’t argue too long with activists for peace, poverty-prevention, environmentalism, or myriad other causes to reach the admission that even if they are doomed to failure, the campaigns must be engaged, because otherwise there is no hope.
A healthier, wiser approach, I’d say, is to shift optimism from the likelihood of one’s personal success to the assumption that reality has inherent purpose and a metaphysical intention for everything to work out in the end. There’s only so much that we can hope to accomplish in the limited spheres of our own personal influence — which seems to be the pessimism that Scruton advises — but the targets of our worldly activism don’t constitute the apogee of profundity.