Polybius was a famous Greek historian who developed a theory of the cycle of governments. He believed that all governments change over time in response to the forces of human behavior. The section below describes his view on how democracies turn into dictatorships.
When a new generation arises and the democracy falls into the hands of the grandchildren of its founders, they have become so accustomed to freedom and equality that they no longer value them, and begin to aim at pre-eminence; and it is chiefly those of ample fortune who fall into this error. So when they begin to lust for power and cannot attain it through themselves or their own good qualities, they ruin their estates, tempting and corrupting the people in every possible way. And hence when by their foolish thirst for reputation they have created among the masses an appetite for gifts and the habit of receiving them, democracy in its turn is abolished and changes into a rule of force and violence. For the people, having grown accustomed to feed at the expense of others and to depend for their livelihood on the property of others, as soon as they find a leader who is enterprising but is excluded from the houses of office by his penury, institute the rule of violence; and now uniting their forces massacre, banish, and plunder, until they degenerate again into perfect savages and find once more a master and monarch. (Book 6 Chapter 2)