This story is from time of mahabharata, when guru drona acharya  was teaching pandavas and kauravas. One day acharya thought of testing his disciples. He called duryodhana  and said, "Do one thing, go and search for a good man and bring him to me."Duryodhana said, "Ok. I will leave right now and start searching for a good man.."After some days, duryodhana came back to ashram and said to acharya, "I searched everywhere, searched far and wide but couldn't find any good man whom I can bring to you."Acharya replied, "Ok. now send yudhishthira to me."When yudhishthira  came to acharya, acharya said him, "Do one thing, go and search for a bad man and bring him to me."Yudhisthira agreed and set out to search. After few days, yudhishthira returned to ashram without bringing anyone with him. Acharya asked him, "Why did you come back alone?"Yudhishthira replied, "Acharya, I have not seen so much evil in any man that I could bring to you."At this, all other disciples were surprised and asked, "Acharya, how is this possible that duryodhana couldn't find a good man and yudhishthira couldn't find a bad man!"Then acharya explained, "When we look at someone, then we look at them the way we ourselves are. then we judge if that person is good or bad.yudhishthira didn't find anyone bad because he himself is good, that's why he was not able to think of anyone as bad.good or bad is just difference of their respective attitudes. you will see what you want to see in a person." Judging others depends on one's thinking.. We do same in our life. We make an opinion about someone beforehand, with out limited thinking and give them title of good or bad. Being good or bad doesn't solely depend on one's nature but it also on our thinking and attitude. 
Our own perspective is also very important in judging someone. 