Why are Europeans so negative toward national identity?

Actually, it is entirely due to WWII. For decades after the war, Europeans had to reflect on the atrocious first half of the twentieth century that was rooted in their culture, nationalism, and ethnic cleansing. Hitler and Mussolini didn't lead small minority fascist parties and conquer Germany and Italy; Germans and Italians embraced them and elevated them to power. Places like Vienna were entirely supportive of Nazism throughout the war. How do you think those people felt about themselves and their culture after supporting fascism into WWII (especially when considering their parents and grandparents supported nationalism into WWI)?

Since the rise of structuralism and post-colonial studies in the 60s, Europe has suffered from cultural self-loathing. This is why Sweden is afraid to push against mass immigration and self-imposed Islamic cultural segregation/superiority issues. Many American liberals, including Obama, have very similar views of American culture and nation. We may very well see in the coming decades an even more rapid cultural decline.
- From New Swedish law criminalizes anti-immigration internet speech