Democracy only works well when people perceive themselves as belonging to one nation. When they prefer to think of themselves ethnically -- Pashtuns in Afghanistan and Pakistan, Masai in Kenya, Uighurs in China -- then democracy simply turns into a contest of who has the most people.
Consider Kosovo, in the news because the International Court of Justice recognized the legitimacy of its vote for independence from Serbia. Kosovo is the old heartland of Serbia, the location of its most important national monuments. For centuries there was an Albanian minority in Kosovo. Since World War II, the Serbs have become modern Europeans, moving to Belgrade and other cities and having small families. The Albanians remained peasants, remaining in villages and raising numerous children, with the result that they now outnumber Serbs in Kosovo. Because they modernized, the Serbs lost their homeland.
Or Tibet. The Chinese government has for decades been encouraging Han Chinese to move to Tibet--where there was always a significant Chinese population -- with the result that some towns in Tibet now have a Han majority. Tibetans are worried that they may eventually be outnumbered in their own land, and this has fed resentment against Han migrants. Already most of the other ethnic groups in China are minorities in their homelands.
Some other solution than majority rule is needed in these situations, but I don't know what that solution might be. Federalism? A defined power-sharing arrangement, like they used to have in Lebanon? Separate ethnic legal systems, like in the old Ottoman Empire? Whatever it is, a world in which politics is simply a contest in breeding can't be our future.