It doesn't always have to be. There are positive expressions of nationalism like rooting for your country in the Olympics, and being generally proud of your country and culture.
But there are specific problems with nationalism as it's often expressed in the USA (and in other countries, but I'll leave those for people who live in other countries). Take Kaepernick for example, he has stated numerous times that his protest has to do with police violence, yet the people who are opposed to his anthem protest say he has to be protesting the troops because he's protesting the anthem. I think people have the right to tell him why they're offended by his protest and they disagree with his logic if they so wish, but I don't think you can honestly tell him "No, you protest isn't about police violence, it's about not liking the troops."
Then we have political debates over the size and expenditure of the military. One side says "We're spending an awful lot on our military, maybe we can afford to make a few cuts here and there." or "I haven't seen any signs of WMDs in Iraq, maybe we shouldn't invade it" while the other side hugs the flag, plays the national anthem and declares the other politician as against the troops and un-American, and even though one side does it more than the other, politicians from both sides will adopt these very stereotypical patriotic optics (saluting the flag, photo with troops, hanging out with old vets, etc..) in their campaign, but fail to deliver when it comes to promises about the VA administration, veteran's mental health access, or the politicians won't hesitate to send them in harms way to score political points.