Why don’t all nations have the same currency?

When somebody from India goes to U.S.A., he cannot use the Indian rupee to buy things there. He has to exchange dollars for rupees. This is true to almost all the countries of the world that, if someone from one country goes to another country he has to exchange the currency of his country with the currency of the visiting country. This is so because every country has its own currency. But why should it be so? Why can we not have just one currency? Every country has to have a corresponding value of gold reserve and other internationally acceptable assets like bonds etc. so as to give the local currency an international acceptance. Currency is generally printed taking into account these reserves and when a country starts printing more currency than its reserve, then the value of the currency goes down. This happens because of the peculiar features of the economy of the country concerned. Since every country has its own economic strategy, hence, it is practically impossible to use one form of currency universally. Though the exchange rates for currency of every country are fixed, yet their value with regard to other currencies changes from time to time. In international trade, every country expresses the value of its goods in its own currency. In fact, the notes and coins are only symbols of the gold reserve of the country. Some important currencies of the world are: Dollar (100 cents) U.S.A. Pound (100 pence) United Kingdom Yen Japan Yuan (100 fen) China Rupee (100 paisa) India Euro Europe Rand South Africa Real (100 centavos) Brazil Currency is just the symbol of the gold reserve of a country.