Probably the most glaring reason would be mankind's natural inclination to compete. War is competition at its best: a man against a man, a nation against a nation, a weapon against a different weapon, etc.. And of course, winning a war or fighting well are grounds for boasting; for men to be awarded medals and to be recognized as heroes. People would no doubt seek the glory and pride that winning a battle would bring.Moreso than that, the spoils of war are riches and land, something im sure noone would be disinclined to gaining.In a sentence, warfare caters to every one of man's inherent desires and thus is so irresistable. Goes my theory anyhow.