While that's usually a long topic with a lot of depth and an oucne of philosophy to it, I'll try to summarize the reason for my beliefs.
Liberalism, socialism, feminism and various ideologies often seem interested in shaping society after a template whereas I find nationalism (and certain forms of conservatism) strives to build a society around a biological reality (such as accepting the de facto differences in say intelligence and behaviour among various groups in society).
A concrete example is that nationalism accepts that a ethnic and cultural homgenity is most of the time beneficial to a nation, as the opposite tends to lead to a more segregated, closed, society in which you don't care as much about the average citizen as you would if you and him had a lot more in common.
The multiculturalism that has plagued the west for roughly half a century now has led to more or less no improvements for the indigenous peoples. The opposite actually; increased criminality, extreme costs etc. But it's not like nationalism is "finished" once a country is ethnically homogenous, or can't exist in a nation without a complex demography, it's essentially politics based around what's beneficial for a ethnic or racial group, in my case the Swedes.