May be you get it from the person who rated you on something, or you do it for yourself, stepping into the shoes of the jury of the act. Either positive or negative its of utter importance in each case, the former helps you boost your confidence whereas the later gives you the areas of improvement.
Not saying that you really need a confirmation for something you believe in, is good for others or helps others in someway, but at some point or the other it feels good to know that what we did actually helped someone in some or the other manner.
Wondering “How negative feed-backs are important? These are words full of criticism, would be totally depressing, how could it be good?” , I’d say, it ain’t depressing, its a glass view of your actions, from others perspective, as in how they saw you doing it, its about what you missed, where you missed and how it would have been better.
Based on personal experiences, there are cases where-in you’ve left no chance of being mediocre or the second best and still you don’t reach where you ought to be.. these cases have a common thing called ‘circumstances’, just keep hitting, you’ll cross it some day and the best part, the next time, you’ll be better than before.. 🙂