It encourages arguments (like us vs. you). It makes others feel superior and selfish. It causes harm to countries and causes violence. A nationalist leader might lie to maintain superiority. Here's an example that might help: America is the greatest country on Earth. Hope I helped!
Nationalism ideology is based in that the individual loyalty to a nation should surpass any other interest. In politics implies identification of a nation with the people. Nationalism can be dangerous as proved by the atomization of the Austro-Hungarian Empire into new small states which went on having serious internal conflicts.
