The reason is that in my opinion it legitimizes war, violence and killing. It promotes patriotism, which is nationalism by another name, and a bus stop short of fascism. Fundamentally it suggests that one nation's people are worth more than anothers'. It's a neat trick, and I have a real problem with that.
This is the same principle as caring about your child more than a stranger's child. A nation is a kin group just like a family is.
Ingroup preference is perfectly healthy when channeled appropriately.
Given how divided some of the most jingoistic 'democractic' countries on the planet right now are (US and UK) I don't think that supposed kinship is what these people get from nationalism.
To me, it looks like an excuse to tribally hate another group or nation without admitting to it.