Instapundit links to a study which suggests that the hormones related to love may also cause war:

"Oxytocin has received much attention for boosting social bonding and cooperation, but it also appears to trigger defensive aggression against outsiders who might threaten an individual's social group, psychologists say. That indicates the hormone has a much more complex role in social dynamics than just encouraging humans to make love and not war."

Move evidence that John Adams was onto something when he said made a similar observation. Against the peace, love, and harmony types of his day, Adams said that love is often the cause of wars.  Adams said that the desire to be seen, and, through that, to attract the admiration and love of others, was the basic political passion.  As he put it in his "Discourses on Davila":

"Who will love me then?" was the pathetic reply of one, who starved himself to feed his mastiff, to a charitable passenger, who advised him to kill or sell the animal. In this "who will love me then?" there is a key to the human heart; to the history of human life and manners; and to the rise and fall of empires.