Why do we fight?

I've been studying about war in my Constitution class. Some really interesting stuff there--like why we call captured enemy "enemy combatants" instead of "prisoners of war" and why we hold them at Guantanamo Bay instead of in the US.
All that's good and interesting, and I hope everyone who wants to have an educated opinion about war would look into those matters.
But I want to discuss something else: what's our motivation for going to war?
With the war in Afghanistan, it's pretty obvious. They were harboring the people who caused 9/11.
With Iraq, it's more complicated.
Democrats say it's because they have oil, or it's because powerful Republicans have connections with the companies that are doing major work there (see Dick Cheney, previous Vice President of Halliburton).
Republicans say it's because of nuclear weapons (now known to be non-existent) or to free people from a dictatorship, or because of links to 9/11 (also known to be non-existent).
Both views are way too simple.
Individuals may have simple reasons for going to war. But the war in Iraq is being fought for many reasons.
It's obviously not just fought because of dangerous nuclear weapons, or to free an oppressed a free people. If that were the case, we'd be in North Korea right now. Unlike Iraq, we know that North Korea has nuclear weapons. They tested one! And conditions in North Korea are horrible. They do a much better job of keeping reporters out, and they do a much better job of keeping their people oppressed, than Hussein ever did in Iraq.
So obviously, something more is going on. There are other reasons for why we're in Iraq (although the fear of nuclear weapons and freeing an oppressed people may have been part of the equation). We ignore these other reasons at our own peril.
So what are the other reasons? I mentioned a few other possibilities (and indeed, Alan Greenspan agrees with the "we're there for the oil" bit).
Why are we in Iraq, and not in North Korea?