A short primer which aims to answer:
Has Iran's influence in Afghanistan changed since the U.S. troop surge in 2010? What steps has it taken in anticipation of the U.S. withdrawal planned for 2014?
Nicely put:
Iran is probably not in favor of a Western-influenced , democratic, affluent Afghanistan, but at the same time, is concerned that an unstable, opium producing and radicalized Afghanistan can also pause a major threat to its interests, as experienced in the 1990s.
Link:http://iranprimer.usip.org/blog/2013...ter-us-pullout

Finally, the author is a former Afghan Foreign MInister.