The total amount of money in circulation, interest rates, inflation rates, international trade situation, demand and supply situation of imports and exports, strength of economy, stability of government are some of the factors that go into determination of exchange rates of respective countries.
The exchange rate of a currency is NOT a signal of its strength v/s other countries' currency. Historically, currencies of strong countries like Japan and Italy have been valued less than Pakistani rupee and is has nothing to do with comparative economic strength of these countries.