Question: Why is nationalism usually taught as a bad thing even though many countries that experience surges of nationalism also have surges of economic growth and technological advancement at the same time?
Answer: This is due to the very superficial, “loaded”, dogmatic propaganda that is ruling public opinion today.
This very cunning, sharply aimed propaganda tries to suppress, delete individual, national identity, trying to create a uniform, easily lead, “zombified” Human mass.
Positive nationalism – not against others but proudly building on traditions, original sources – is a constructive force as you suggested.
Moreover individual, national identity is “burnt into our DNA”. It can’t be suppressed, deleted and the more the above mentioned, shortsighted ideology tries to do this, the more violent rebound reactions it will evoke which might burst into negative nationalism, fascism as we can already expect it in Europe.
We need to learn how to recover our healthy, positive individual and national identities and then also learn how to put together a united, mutually responsible and mutually complementing mosaic of Humanity from its diverse parts that exactly know what they can contribute to the whole!