Why would a person constantly find fault in a person?

I'd say it would be more on the personal side of the person who is finding the faults. A person might constantly find faults in a rapper because he hates the style of music, or more emotional such as hating gay people because of something that happened in someones past. A kid might find faults in a kid he hates at school because of a fist fight that happened between the two. ETC.