Nationalism and patriotism are contemplative feelings of continuity. While imperialism and chauvinism have brought more destruction than contribution to civilisations, without nationalism and patriotism the arts and letters wouldn't exist. Let's take, for example, any of the great creators. We see from their work that they were very much rooted in their present and past and that national history and tradition were a main source of their inspiration. Even the modernist innovators needed tradition to know what they were breaking away from.
Nobody is a blank page.
Of course any overdose may be dangerous, even vitamins. Therefore our intuition and knowledge should keep us from sliding too far, meaning from nationalism and patriotism into chauvinism.