I know very little about Iran, other than the headlines I read in the mainstream US press. I bet you don't know much more than I do, though like me you are probably cheering for the "underdog" Mousavi to beat that awful Ahmadinejad. But if I reflect honestly, I have to admit that I don't really know what's best for Iran, or even the long-term interests of the United States. I cheer for one side because it's "my" side, and because a lot of people I like are on this side.
This is no different than why people cheer for a particular sports team. A Mariners fan doesn't care about baseball in any purely detached or objective sense. He wants his side to win because it's his side. Although you could imagine an objective standard of "Truth" about which team is ultimately the best, even the most well-informed sports nut -- the guy who can recite statistics all day -- is going to cheer for his team, not because it's the "best" but because it's "his". It's not about truth, it's about entertainment.
Most well-informed political junkies are the same: to them, politics is a form of entertainment. It's not about being "right", it's about cheering for and supporting one side, sometimes for no reason other than to oppose the competing side. Sure, they can recite facts and statistics -- they enjoy it! -- but press them on why, or about the truth of the matter, and it comes back to "because my side says so".
I think entertainment gets in the way of truth. Few of us have the time to dig into each policy decision in the kind of detail necessary to come to a real opinion, so it's nice to delegate our thinking to a political party. But combine that with the natural tendency of Type A people to dominate conversations, and we get cacophony. No real understanding, just a bunch of loudmouths who classify everything as either Republican or Democrat.
I'm trying to think of a good answer for the next time somebody asks me my political party. I find that the question itself is like asking my favorite baseball team: it's not about any serious discussion of Truth or the issues, it's about figuring out which (of presumably only two -- why is that?) team I'm cheering.
Libertarians aren't really a political party as much as a mindset. Ask what matters to me in the current situation with Iran and, without knowing any facts, I'll give you as reasonable an answer as is possible without a lot of research. But, like domestic political issues, why do I have to cheer for one side or another?