reasons: Iran supports Assad in Syria, Hezbollah in Lebanon, both enemies of Israel; and the Houthis in Yemen, because their leaders are at least nominally Shi'i. Iran also appears to be supporting dissent and occasional rebellions by Shi'is in the gulf countries and in Saudi Arabia.
The Saudis may also oppose Iran because their own brand of Sunni Islam is intolerant of Shi'is, though it is doubtful that the leaders are very religious.
Israel may have reason to fear the development of Iranian nukes, but consider that they were also quite opposed to Pakistan acquiring nukes, yet they have not represented Pakistan as a significant threat since it became a nuclear power. I surmise that this is because the primary use for nukes is in protecting countries against attacks, not in their actual use. That is why Saddam Hussein wanted the world to think he had them or was on the verge of developing them and that is why North Korea does not want to get rid of theirs.
The more vexing question is why the US should have a dog in this fight at all. I suspect that it is because, like Saddam Hussein and Qaddafi, Iran may represent a threat to the petro-dollar, which is the basis for the use of the US dollar as a reserve currency.
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