The sizeable Kurdish community in Syria are a complication. The Turks don't want to supply and train Syrian opposition only to end up as the victims.
Ultimately the Kurds would like to have their own country carved out from northern Iraq, a chunk of Turkey and a chunk of Syria. Just now in Syria they are asking that any post conflict settlement is a federal state with a semi-autonomous Kurdish state within a state (much like they have achieved in Iraq).
The Turks are in the middle of the process of coming to a political settlement with the Kurds. They have Abdullah Öcalan in jail, which makes him very available for negotiations.
Lots of information about the various political machinations here:
ORSAM