I would say your best answer is "human nature."
In Federalist #10, James Madison wrote, "The latent causes of faction are thus sown in the nature of man." He argued that it is human nature that because people have different opinions, different amounts of property and wealth, and have other differing interests, they will align themselves into factions or groups that have interests similar to their own. Then these various factions will battle each other for advantage. This tendency of human nature is the problem of factions that needs to be controlled in civil society, and the subject of Federalist #10.
