image from: http://www.ishmaelscorner.com/2013/04/23/pr-answer-classic-storytelling-arc/
Related to story arc(LINK??)
Our minds have trouble processing the same thing over long periods of time. The same way a story must ebb and flow to keep our attention, slowly working its way towards periods of action and the climax, games must do the same with arousal.
Think of a horror movie. If the entire movie, with no break, was at level 100 gore and screams, no one would be able to watch it. It is just as important to have lulls in the action as to have excitement.
This can be exemplified in sports. For example, baseball. Much of the game is very slow, the players are waiting for the moment when a ball gets hit, and they must jump to action. (Either to run around the bases, or to rush and catch it.) No single player is expected to be in action (Rushing around, running very hard) for the entire game. First of all it would be difficult physically to keep up with that kind of pace. Secondly, those moments of excitement (High arousal) wouldn't be as special if they were not punctuated with slow sections (Low arousal).
Baseball is an interesting example also, because many people find it to be a terribly boring game to watch. For some people the peaks are too few and far between and most of the game feels as if it is in a valley. Games like soccer/football keep watchers attention better because somewhere on the field there is always something very arousing happening. (Though it is divided among players, so no player must be working at the peak of their arousal the entire game.)
Games are most successful when there are both periods of excitement and relaxation.
When designing with this in mind, you should ask yourself: