I dislike the idea of world peace, it's boring, if not unattainable. How exciting is a world in which there is no struggle? No fighting? No tragedy? barely entertaining in my opinion.
Besides, struggle makes people stronger. The more a person struggles and overcomes it the stronger they become and a world with "world peace" would have little conflict and so some people would not advance. However in a world with struggle and fighting, people will suffer, they will strive for victory, and they will die and those who are victorious shall be the better and shall advance while those who do not shall fall.
War and conflict are interesting things, with war, the more conflict that occurs the more people and especially technology advances. While with conflict it can be individual or it can be groups which become stronger over another.