It is clearly not easy for man to give up the satisfaction of this inclination to aggression. They do not feel comfortable without it. The advantage which a comparatively small cultural group offers of allowing this instinct an outlet in the form of hostility against intruders is not to be despised. It is always possible to bind together a considerable number of people in love, so long as there are other people left over to receive the manifestations of their aggressiveness. I once discussed the phenomenon that is precisely communities with adjoining territories, and related to each other in other ways as well, who are engaged in constant feuds and in ridiculing each other — like the Spaniards and Portuguese, for instance, the North Germans and South Germans, the English and Scotch, and so on. I gave this phenomenon the name of “the narcissism of minor differences”, a name which does not do much to explain it. We can now see that it is a convenient and relatively harmless satisfaction of the inclination to aggression, by means of which cohesion between the members of the community is made easier.
The Narcissism of minor differences
http://en.wikipedia.org/wiki/Civilization_and_Its_Discontents