Sunni and Shia Muslims

I was hoping to find out if anyone knew why Hamas and Hizbollah are not in conflict and why Hezbollah was being supported by Syrian President Bashar al-Assad who is Sunni? If Sunni and Shia have conflicting interests (even to the point where the Shia were violently oppressed by Sunni in countries like Iraq and Pakistan) that they would support each other?

Please feel free to correct me if I am wrong or being ignorant about this as it has me a little confused?

Does anyone know where I might be able to find out.. I have googled till i blistered but found nothing..