Democracies are temporary in nature; they simply cannot exist as a permanent form of government. Democracies can exist until voters discover that they can vote themselves generous benefits from the public treasury. From that moment on, the majority votes for the candidates who promise the most benefits from the treasury, with the result that democracies finally collapse due to loose fiscal policy and deficit spending, which is followed by an economic depression and a period of political anarchy or despotism.
This quotation is often attributed to Alexander Fraser Tytler, apparently incorrectly, but nobody seems to know who really said it. Whoever he was, he was a smart man.