Why must countries trade with each other

Countries trade with one another to receive a variety of items, therefore, the markets have many different brands to sell, and the customers have many choices. Also because some countries do not have the resources and need them, so they trade some of their unique resources with another countries resources. 

Countries must trade with each other in order to gain resources that they don't have. Look at petroleum for an example. Americans have excavated all the oil that's in the states, but since we rely so heavily on gasoline to simply function, we buy and trade with the middle east for the petroleum they have there. That's why its so important to keep good relations with the Middle East simply because we rely so heavily on them for this good.

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