What exactly is the logic behind nationalism?

You can convince anyone of anything when you raise them from birth surrounded by the message you want them to receive. The logic behind it is obviously law, order, obedience, and control. Making children recite the Pledge every day at school, decorating everything in your country's colors to increase branding and "team spirit," a constant stream of propaganda insisting that ones country is inherently better than every other on earth.... It all serves to create an atmosphere where it "feels good" to obey and "it's the right thing to do." Questioning this gets you labeled "anti-American" and that's so "bad" that you might as well be called a "terrorist." It's a pretty well-designed system.