But even from an entirely selfish point of view, there are reasons to care about economic prosperity in other countries - it allows them to import more of your products, keeps the people there from trying to emigrate to your country or growing resentful at your country, and so on.

In any case, if you want to go that entirely selfish road and don't care a fig for what happens in other countries, fine, as long as you admit it out loud and don't complain when other countries return the favour.