In the game of poker, opens and raises are considered aggressive plays, while calls and checks are considered passive (though a check-raise would be considered a very aggressive play). It is said that “aggression has its own value”, meaning that often aggressive plays can make money with weak hands because of bluff value. Aggressive plays also tend to give the opponents more opportunities to make mistakes.
While it is true that aggressive play is generally superior to passive play, using any play exclusively can lead to predictability, and being too predictable is far worse than being too passive. A player who is constantly aggressive and plays many inferior hands is called a “maniac”, and skilled players will take advantage of him by calling him more often, using isolation plays, and by other means.
If a player is not aggressive with his weaker hands, the opponents can safely fold whenever the player does bet or raise. The appropriate amount of aggression can be computed using game theory, and depends on the game being played and the tendencies of the opponents.