Nationalism is a by-product of ignorance and fear towards people of different cultures. With increasing education, and a focus on unity of the people rather than unity of "the nation", we can eventually eliminate the hideous demon that is nationalism. It shall take time, but a socialist utopia cannot be built in a day.
'Tis a by-product of those perhaps, but also of a sense of camraderie, like a sports team. One wishes to see one's own team win. That is what you were hoping when Russia attacked Finland, was it not? You did not fear and hate the Russian, you simply wanted the VSVR to be victorious.