I believe it is about a father trying to explain to his son that he is going off to war, to kill people. Now, saying that in those words is very blunt, so he avoids the explanation by saying, son, someday you'll understand, and i think the point the song is trying to make, is that it is hard to find a real and plausible reason for war in the first place. So, i think that by the father telling his son, someday you'll understand, he's implying that he doesnt really understand it himself, hence, "someday never comes".

-hope that made any sense