What do you think makes a country's currency go up or stronger?

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It sometimes fascinates me or makes me wonder what really makes a currency go stronger against other currency or against the dollar? Any idea? Is the gross national income of the country or the political stability and national security? what really boost it up. And how come the value of currency of a certain country is low eventhough the country is well developed and produces great income, like Japan, its currency is low compared to the dollar but the country is productive. Any idea?