With this thought pattern people see the world in absolutes: everything is either all good or all bad, positive or negative. For example, a person with leg pain that prevents her from walking very much might say to herself, "Well, I can't walk my normal amount, so I won't walk at all." This way her health deteriorates further. Taking a more moderate or balanced view of her life would allow her to set intermediate goals and work towards gradually improving her situation.
(Warning: This entry is part of Negativity Week!)