It is well understood that voters, by and large, vote irrationally: emotionally, on the basis of "buzz" instead of facts, and inconsistently. (See this, this, and this, for example.) Voters are prone to vote against their own long-run interests because they do not understand the consequences of the sound-bite policies advocated by politicians (nor do politicians, for that matter). American "democracy," by indiscriminately granting the franchise -- as opposed to limiting it to, say, married property owners over the age of 30 who have children -- empowers the run-of-the-mill politician who seeks office (for the sake of prestige, power, and perks) by pandering to the standard, irrational voter.
Essentially, then, democracy is an enemy of liberty, as I have explained several times in various ways:
Democracy vs. Liberty
Yet Another Look at Democracy
More about Democracy and Liberty
If Liberty Depends on Democracy, We're Doomed to Slavery
Conservatism, Libertarianism, Socialism, and Democracy