Proud of our rights and freedoms, our national identity, our beautiful land, our power, our national accomplishments (the moon, our inventions, winning wars, world's largest economy, first free society, etc)
That's why people are proud of the flag. It's not just white people. I live in a neighborhood with many asians and I see them with American flags. I see people of all races with American flag/pride t-shirts, hats, etc. Why put a race to the idea of national pride?