In cooperative games, players have 1 common enemy/objective to complete. A game with no adversaries or obstacles would be unchallenging, which is why we usually find some kind of Artificial Intelligence within a cooperative game, the one with a sole purpose of challenging and stopping players from achieving their purpose.
In video games, AI are usually built with a set of coded behaviour and ran automatically by the program (the game), sometimes creating an illusion of free willed action, but in board game it’s a different case altogether. The common enemy in cooperative board game are so far from the definition of intelligence, it will be far more suitable to call it Artificial Reaction or Artificial Law. It is merely a set of rules of cause and effect that is set within the whole game flow to create a certain kind of situation in response to players’ action.
When I was working with one of my board game, I had a playtest session with a group of friends, and the funniest reaction I ever get was the fact that one of my friend complained and said that he felt like he was being deceived and bullied by me. It was an honest reaction and one that make me realize that not all people can actually came into such conclusion if they don’t actually see the game designer when they are playing. And that reaction made me realize that the whole game rules was not sufficient enough to engage the players.
When players are enjoying their time, they will become unaware that the game they are playing is actually a set of artificial law, the kind of law that have no actual effect (think of genuine law such as law of physics, the ones that actually have real impacts on you), and yet players are willingly doing their acts in accordance with those laws and rules, just because it makes sense and they don’t feel like they are being cheated or bullied around.
Therefore I believe the sense of fairness within a game rule is the most important key in creating an engaging game, one that can be achieved only by considering players’ wishes and expectations.