"a pure democracy can admit no cure for the mischiefs of faction. a common passion or interest will be felt by a majority, and there is nothing to check the inducements to sacrifice the weaker party. hence it is, that democracies have ever been found incompatible with personal security or the rights of property; and have, in general, been as short in their lives as they have been violent in their deaths." explain

If power is rooted in the will of the majority, there is nothing to stop the majority from looting and oppressing any minority. Since there will always be more incompetent and lazy men than brilliant and hard-working men, it follows that sooner or later the majority will simply vote itself "bread and circuses" paid for by the productive minority, and the state will collapse, usually resolving into dictatorship or (in the past) conquest by a better-run nation or group.

You can observe this on a small scale in school by observing what happens in a large "group" project: quite often you find only a minority of people working as hard as they can, and a number being willing to take advantage of the efforts of others in a way they would NOT if their own grade was personally on the line. (Many teachers award individual grades even in group projects for just this reason, to forestall this.) You may also observe that a house shared between adults, and rented, is messier and less well-kept than a house owned by a single adult. Same thing: if people can rely on others to do the hard work, and share the fruits, they often will -- often enough to make any kind of collective ownership a routine disaster for humanity. That's just the way we are.

This problem was well-known in the 18th century, and one reason why democracy and republicanism had kind of a bad reputation. Enlightened men tended to favor absolute monarchy, perhaps limited by a powerful aristocracy and/or parlaiment, because (1) a single ruler was limited in the damage he could do; his appetites no matter how outrageous were far less dangerous than the appetites of millions; and (2) he could represent a consisent bulwark against the fads and impulses of the majority. He would necessarily be sensitive to them, since he probably didn't want to end up with his head on a pike, but he would resist them when they were unusually foolish or strange and probably short-lived.

The solution argued by Madison is that a *large* republic would have so many factions that they could never get together and agree on *which* minority they would exploit, and so there would never really be a consistent long-lived parasite majority that could destroy the country by exploiting some productive minority. It's unclear how right he was about that.