Cutting the long story short – information. Information causes  changes on every single financial market. However in reference to the currency market, information of a special nature is relevant. In case of the Forex, we are dealing with a huge amount of information that influences each other and shape the current relationship of currency values. There is nothing strange about this, Forex is based on determining a relative relationship between two different currencies. Therefore, in dealing with only one pair, you should take into account a situation within two different economic organisms. However, bear in mind that the economy of unrelated countries with your pair  may also have an impact on your trade.