A game is a series of interesting decisions.
If indeed Sid Meier is right (and who would challenge his authority?) then designing a game is about designing decisions, and how can we design decisions if we don’t understand how they are made? Classical game theory assumes players are unswervingly rational in the pursuit of their goals, and perhaps some games (such as chess) promote such a mode of thinking, but in general we are not nearly so logical. We make mistakes, we overvalue certain alternatives, we are dishonest and we give in to social pressure. Moreover we fail to recognise these failings even when we repeat them regularly.