Why are net exports and net capital outflow always equal?

Net exports and net capital outflow are always equal because every international transaction is an exchange. When a seller country transfers a good or service to a buyer country, the buyer country gives up some asset to pay for this good or service. The value of that asset equals the value of goods and services sold. Hence, the net value of goods and services sold by a country (NX) must equal the net value of assets acquired (NCO).