Like any other country; Iran will do what it has to in line with its national interests; regardless of whether others approve or not. Saudi and the Gulf states do certain things that are detrimental to Iranian interests - it works both ways. Granted the Iranians weren't entirely blameless but they haven't forgotten the devastating 8 year war fought against Saddam who was financed by Gulf petrol dollars. Way before September 11th 2001; Saudi dollars played a big part sustaining the Taliban and even after September 11th Saudi dollars was making its way into Afghanistan. It was Iranian, Indian and Russian assistance that kept the Northern alliance going. Without the Northern Alliance the U.S. would have found it much harder gaining a foothold in Afghanistan.
Saudi and the Gulf states should putting all the billions spent on their armed forces to a useful cause by assisting the Iraqis defeat IS; instead they'd rather intervene in Yemen where they have accomplished zero. On the contrary I'd argue that the Saudi still have lots of friends; that what petrol dollars enable them to have : ''friends''. There is a strong pro- Saudi lobby in the U.S. and dozens of countries going out of their way to ingratiate themselves with Saudi Arabia. For the Saudi and the other Gulf states the ''heretic'' Iranians are ''hostile but the Iranians can also provide legitimate reasons why the Gulf Arab states are themselves ''hostile'' to Iran. The Saudis were so disappointed that Obama refused having a go at Iran's nuclear facilities to the extent that they reportedly agreed to Israel using Saudi airspace to reach Iran; now they're hoping that Trump will do it.
Mr. Strum, Saudi policy has always been until Obama an extension of US policy and not the other way around. The Saudis see that:
1- The awakening of Catholicism in Europe and Islam in central Asia was a US policy to undermine the soviet union. Saudi Arabia was financing on behalf of the CIA x "Afghan freedom fighters" nowadays "Terrorists".
If you go back to the history books you will notice that in exception of a brief period in World war 1. Western Europe (Spain and Italy excl.) had always supported the caliphate against central Europe and the Russians.
2- Pakistan and India separation was favoured by the Brits and Pakistan was later protected by the Americans via Saudi financing.
3- The division of Pashtuns between Afghanistan and Pakistan was a line drawn by the British. The Northern borders were a line of separation between the British and the Russians. Afghanistan is inherently unstable because it doesn't have many Afghans in it but rather a combination of different Turkic and Persian tribes.
4- Iran-Iraq war saw western support for Iraq to weaken a highly armed nascent anti-american Islamic revolution. They had killed and kidnapped many French, US etc. nationals in Lebanon and Iran. At the same time the west supported Iran (Iran gate) to prevent a total collapse and maintain an equilibrium but with a weaker Iran.
5- The US had the choice of alliances in the middle east with a vast array of secular and nationalistic movements. Egypt's Nasser looked for an alliance with the US but was refused and had to resort to an alliance with the soviets to industrialise Egypt. The US preferred to ally itself with the Saudis at that time.
6-
Salafi Sunni Saudi Arabia supported US policy in Yemen against Nasser who was fighting against a
Shia Mullali government to establish a secular government.
7- Saudi Arabia has never officially sponsored any attack against US interest unlike Iran and Iran affiliates who engaged in Embassy burning, westerners kidnapping and killing campaigns.
8- The sunni insurgency in Iraq was armed by the Syrian (Iran proxy) and were entering Iraq from Syria.
9- There is a distinction in Saudi Arabia between the ruling family and the clerics. King AbdulAziz had resorted to Wahabi ideology to unite historically at odd tribesmen in the name of religion to form Saudi Arabia. He later fought them (supported by the British) and won over them as they wished to continue their Jihad at that time against British protectorates and beyond. Ever since there is a distinction and a balance in Saudi between the two. Saud family is in a position to declare the return of the caliphate that is against western interest but has never wished to do so to the contrary it has fought the Muslim brotherhood and others.
10- Saudi is not arming opposition in Iran, but Iran is actively arming Shia and Sunni opposition in Saudi, GCC and Arab world.
As you notice Saudis were always financing western policy in the Muslim world and beyond. Of course they will feel betrayed when they see Americans siding with the Persians even in Yemen!