Question: Based on what criteria sometimes we give up on things?
Answer: We are basically very “simple” creatures, as all of our actions are based on an egocentric “pleasure/pain” calculation. We move, act either in order to receive more, greater pleasures for ourselves, or because we want to distance ourselves from expected or actual pain, suffering.
Whether we “give up on things” depends solely on the importance of obtaining those pleasures/ escaping the pain suffering.
Thus if the pleasure we want to receive is greater than anything else we can consider in life, we keep on chasing it until we get it against all odds. On the other hand if the pain or suffering is intolerable, if moving away from it determines our survival, we will keep moving until we escape at all cost.
So the criteria you are searching for is the level of importance for acquiring a desired pleasure, or escaping a threatening pain, suffering.
Entering The Force-Field 