What causes factions?

Madison defines factions as groups of people who work together to protect and promote their special economic interests and political opinions. Although factions are most often at odds with each other, they all work together against public interests, infringing upon the rights of others.

According to Madison, factions are inevitable. He says that as long as men have different opinions, different amounts of wealth, and own different amounts of property, they will continue to both socialize and fratenize with people in similar circumstances.