Often, a player would like the hinder the progress of another player. Or, two or more players would like to work together to accomplish a common goal. In order for diplomacy to emerge, the game must permit this PlayerInteraction.

The article I_Have_No_Words_&_I_Must_Design? sums up it's argument for diplomacy with this statement: "How can players help or hinder each other? What incentives do they have to do so? What resources can they trade?"