Randolph M. Nesse:

"When people are making progress toward their main life goals, they feel fine. Obstacles provoke frustration, often observed as anger and aggression. Inability to make progress toward a goal causes demoralization and temporary withdrawal. Prolonged failure of a strategy leads to more severe demoralization and attempts to find alternatives. When extended efforts fail to find a new route to the goal, intense low mood disengages motivation from the goal. When the unreachable goal is truly given up, low mood is replaced by temporary sadness aroused by the loss, and the person moves on to pursue other more reachable goals."