In this quote from The Federalist Papers, James Madison explains why pure democracy can be a bad thing:
…in all legislative assemblies the greater the number composing them may be, the fewer will be the men who will in fact direct their proceedings. In the first place, the more numerous any assembly may be, of whatever characters composed, the greater is known to be the ascendancy of passion over reason. In the next place, the larger the number, the greater will
be the proportion of members of limited information and of weak capacities.
Now, it is precisely on characters of this description that the eloquence and address of the few are known to act with all their force. In the ancient republics, where the whole body of the people assembled in person, a single orator, or an artful statesman, was generally seen to rule with as complete
a sway as if a scepter had been placed in his single hand.
On the same principle, the more multitudinous a representative assembly may be rendered, the more it will partake of the infirmities incident to collective meetings of the people. Ignorance will be the dupe of cunning, and passion the slave
of sophistry and declamation. The people can never err more than in supposing that by multiplying their representatives beyond a certain limit they strengthen the barrier against the government of a few. Experience will forever admonish
them that, on the contrary, after securing a sufficient number for the purposes of safety, of local information, and of diffusive sympathy with the whole society, they will counteract their own views by every addition to their representatives. The countenance of the government may become more democratic, but the soul that animates it will be more oligarchic.
The machine will be enlarged, but the fewer, and often the more secret, will be the springs by which its motions are directed.
-Madison, Number 58