Some notes on the Iranian support for the syrian regime:
1. First of all, Iran does not support Assad as a person. Iran supports the Syrian regime which is and has been a decade long ally of Iran in the region.
2. Irans motives are neither (a) “expansion of Shiism” nor (b) “suppression of Sunnis”
Had it been (a), Iran would forcefully (try to) “shiitize” the non-Shia parts of Iran.
Had it been (b) Iran would not have supported Sunni Palestinians, Sunni Bosnians or the Sunni majority “Northern Alliance” in Afghanistan. Plus, Iran in the 80s accepted millions of overwhelmingly Sunni afghan refugees while itself being at war with an arab country that was strongly supported by wahhabi Sunni states Kuwait and Saudi Arabia.
Reflecting on these FACTS anyone should be able to dump all the nonsensical trash regarding Irans alleged plan to “exterminate Sunnis” (as Yusuf al Qaradawi ridculously claimed)
3. Irans support for Syria is a defensive action. Iran did not out of the sudden send material and later men to Syria to fight an offensive war for territorial expansion or ideological export. Irans actions are an attempt to save a regime that is Iran friendly and that has a frontier with Israel but – contrary to Jordan – has not signed a peace with Israel.

To counter the constant Israeli threats Iran needs Syria as a transit land for weapons deliveries to the Hezbollah.
Now, Hezbollah is neither willing nor capable to seriously attack Israel or “liberate” Israeli territory. Instead Hezbollah has 2 roles:
a) defend the mostly Shiite southern Lebanon from getting an Israeli buffer zone again (where Palestinians could be exiled or mass deported)
b) be Irans “Joker” in case Israel attacks Iran. It is a totally different issue whether Israel has to take into account at least medium damage and casualties resulting from Hezbollah counter attacks or whether Iran has no militia on Israels doorstep to retaliate for Iran that itself lacks a common border with Israel and the necessary long range aircraft to reach that country.

(b) also applies to Syria and even on a much broader scale. Syrias military, militias etc. have by far more man power and better weaponry than Hezbollah. Syria offers much more territorial depth for Iran to deploy parts of it airforce there to bridge the distance to Israel.
Even though technically inferior to the Israeli airforce a medium size fleet along with missile defence systems around and in front of it starting from syrian soil towards Israel would leave Israel less time to react.
Iran could also place ground forces and short range artillery in the Golan area to accompany possible aerial attacks with ground attacks.
All this would not be enough to defeat Israel but it would be enough deterrence to let Israel think twice before attacking Iran for no valid reason.

So, these are the reasons why Israel and the US and at least indirectly UK and France are so keen on regime change in Syria. They know exactly that Assads removal would severely weaken Hezbollah.
Saudi Arabia and Qatars motivations for regime change are clearly the following:
– weaken Iran
– denounce and weaken Hezbollah, a Shia movement that enjoyed popularity and support among many non-Wahhabi Sunnis
– Secure further american support in case they have to encounter their own “arab springs”

Being awash in money and lacking military man power the Gulf arab countries run a huge defamation campaign against Iran and it´s arab allies by inventing the legend of an alleged large scale campaign to marginalize, suppress and kill Sunnis simply because the are Sunnis, as though Assad, Hezbollah and Iran were leading a religious war, an inner-islamic crusade against the majority of Sunnis who outnumber the Shia 15:1.
That this is pure trashtalk is obvious to anyone who is able to think without religious fervor, but some arab audience, especially in poor, socially neglected rural areas with unemployed youth suffering from low or no education proves to be receptive for such propaganda.