Learned helpnessness : Why do we stop trying?

Learned helpnessness is a psychological phenomenon that turns otherwise healthy individuals into passive, lethargics, after going through repeated bout of stress over whose cause they  don't have the slightest control.

It pushes people to develop a sense of impotence and a meek resignation that verges on fatalism, even when the situation changes for the better and opportunities for change present themselves.

When the stressor is removed, or when it could be easily controlled, people still exhibit a significantly reduced effort to improve their situation.
There is a story about an eagle who suffered an injury that required several months of healing before he could fly again.
He was placed in a "bird clinic" which had low-ceilinged structure designed for "ground" birds. 

The eagle would repetitively try to take flight, only to bump into the low mesh ceiling. 

After weeks of failed attempts, the eagle gradually resolved to accept the status quo and stopped trying to fly altogether.
When The eagle was fully healed, he would only fly from perch to perch, never attempting to fly higher than that.

The eagle was moved to a massive aviary enclosure with a higher ceiling, but it wouldn't fly as higher as the other birds.
It learned from months of failed attempts that flying led to failure and pain. 

It had developed a "learned" helplessness.