There are two main reasons for this.
First, democracy is the form of government that best protects the rights of the individual. People have the right to participate in ruling themselves. They also have right to such things as free speech. Democracy is better at protecting these rights than any other system of government.
Second, democracy uses the "wisdom of the crowd." The idea here is that any one person can be spectacularly wrong, but the people as a whole are not likely to make horrible mistakes. A form of government that allows more people to participate is more likely to come up with the right policies than a form of government that allows fewer people to be part of the decision-making process.
For these two reasons, democracy is both the most just and the least dangerous form of government.