If taken too far, nationalism causes double standards where all non-nationals are considered lesser people- subject to all manners of evil at the hands of nationals.
And when taken too far it ends up tied to a religion, race, or ideology... and attempts to force a heterogeneous culture.
Nationalism is an ideology that puts the collective over the individual, losing sight of the fact that society is nothing if not for individuals. And it sets arbitrary values to humans based on geographic location.

Loving your country is a good thing.
Making the nation your primary identity is another.
People are more free when they are individual humans. The nation-state is our toolbox, not our cage.

__________________
"Conservatives say if you don't give the rich more money, they will lose their incentive to invest. As for the poor, they tell us they've lost all incentive because we've given them too much money."

-George Carlin