I don't have a problem at all with it being voluntary.Nothing wrong with nationalism to a certain degree, doesn't mean you need to be xenophobic or a believer in ethnic nationalism. Thats two things that I dislike with a great passion.
Nationalism is a word with a negative connotation, but it doesn't have to be a negative thing imo. If you believe your nationality is superior to others, or that your etnicity sets you above others by default then yes thats something that is very negative imo, feeling like you should contribute with your time for a limited period of time for the better of your country is not a negative thing though. Esp since you may very well gain something quite beneficial from it as an individual as well.