Withdrawing troops out of Iraq would mean US would have more military resources available to them should they need them in dealing with Iran. It might not be wise having troops scattered in 2 countries in that region. If war were to happen in Iran, US troops might be in more danger if they are still in Iraq.
I support the United States withdrawing from Iraq and allowing the legal government of Iraq to run their country for many reasons. The possibility of improving relations with Iran is not one of those reasons.
Both the Iran government and the U.S. government know that the issues that the two nations have with each other are not related to the issue of Iraq. The only thing that will improve relations between the two nations is Iran holding to the details of the Geneva interim agreement on the Iranian nuclear program. This is the moment where Iran can prove whether or not they are a nation that can be reasoned with.