Why do people do this?

It seems to be that in settings such as yahoo answers, people are more likely to 'thumb down' other answers more than they are to 'thumb up'. Why is that? Do people just want their own answers to seem better by throwing other ideas under the bus? I mean if you were an OP scrolling through ten answers and you saw that an answer had like 4 negative votes, you probably wouldn't even read it or would have some potential bias based on what the general consensus appears to be. Many times too i hear people describing other people's flaws before they praise their positive attributes; why?

All in all, and in life in general perhaps, why do you think people judge others harshly or negatively?