Democracy is often about choosing a power, a ruler or a reign, choosing a political party to have all the power of a country, and while that probably is better than a fascist state where a ruler brutes or bribes himself to power without being elected, it still is choosing someone else to rule and decide for you, which doesn't make that much sense from an anarchistic perspective.
If you talk about democracy in another way. Let say that the public votes for some political decision through a non-power society, well its a bit different, and perhaps depends a bit on the issue.. but in most cases it still makes no sense that a majority of people should decide over everybody. Its still kind of statist. Anarchists usually believes in freedom to the individual, and not in the support of controlling others.
Being ruled by others just because they're in a majority makes little sense. Kind of a "argumentum ad populum", really.