First, as the vast majority of money is issued by private banks – bank deposits – there can only be a single currency if there is a single banking system. For money to be truly one, it has to be truly fungible independent of its form and independent of its location.
That requires stronger common governance of the banking sector in countries that share a currency, so that bank deposits inspire the same level of confidence wherever they are located. Second, sharing a single currency heightens the degree of mutual vulnerability to which member countries are exposed.