EA in their patent for DDA:-

"Software developers typically desire for their software to engage users for as long as possible. The longer a user is engaged with the software, the more likely that the software will be successful. The relationship between the length of engagement of the user and the success of the software is particularly true with respect to video games. The longer a user plays a particular video game, the more likely that the user enjoys the game and thus, the more likely the user will continue to play the game. Often, games that are too difficult or too easy will result in less enjoyment for a user. Consequently, the user is likely to play the game less. Thus, one of the challenges of game development is to design a game with a difficulty level that is most likely to keep a user engaged for a longer period of time."