There are at least two reasons why nationalism might cause a country to want to industrialize its economy. To see why this is so, let us think first about what nationalism is. Nationalism can be defined as a feeling of intense pride in one’s country and the feeling that one’s country is superior to other countries.
If you feel that your country is superior to others, you might want to prove it by making your economy stronger through industrialization. In the modern world, countries with industry are typically richer than those which have not yet industrialized. Countries that have not industrialized rely more on things like farming and fishing and do not have very modern economies. If you think your country is superior, you will want it to have a superior economy. That requires industrialization.
If you feel that your country is superior to others, you might also want to be militarily stronger than other countries. Industrialization can help with this as well. If your country is industrialized, it can produce things like weapons instead of having to buy them from other countries. (This is becoming less true, however, as weaponry becomes more and more sophisticated.) If your country can produce the weapons and uniforms and other things that its military needs, you can be prouder of your country than you would be if your country had to buy these things abroad.
For these two reasons, nationalism might lead you to want to industrialize your economy.