From Feeling Good: The New Mood Therapy by David Burns
You pick out a negative detail in any situation and dwell on it exclusively, thus perceiving that the whole situation is negative. For example, a depressed college student heard some other students making fun of her best friend. She became furious because she was thinking, "That's what the human race is basically like--cruel and insensitive!" She was overlooking the fact that in the previous months few people, if any, had been cruel or insensitive to her! On another occasion when she completed her first midterm exam, she felt certain she had missed approximately seventeen questions out of a hundred. She thought exclusively about those seventeen questions and concluded she would flunk out of college. When she got the paper back there was a note attached that read, "You got 83 out of 100 correct. This was by far the highest grade of any student this year. A+"
When you are depressed, you wear a pair of eyeglasses with special lenses that filter out anything positive. All that you allow to enter your conscious mind is negative. Because you are not aware of this "filtering process," you conclude that everything is negative. The technical name for this process is "selective abstraction." It is a bad habit that can cause you to suffer much needless anguish.
It kills me that we can so effectively filter out positive events that we believe that negative is our only experience. I know that I do this sometimes when I get down-- and it still amazes me that it actually happens.