Israel allows the U.S. to maintain a foothold in the Middle East and have an influence in the region (which is plentiful in natural resources like oil), that’s the gist of it. The U.S. also has a large Jewish community that is generally pro-Israel (not all of them, but many are. In recent events, many Jewish groups in America are now condemning Israel for their war in Gaza, starving the population, all that heinous stuff)

Iran’s government has made it clear that they don’t like Israel’s existence (due to their displacement of Palestinians, various war crimes, that sort. They also don’t like that they are so heavily American influenced). They don’t like the U.S. either, as they believe that they are an imperialist power that wants to take advantage of the Middle East. That is one reason that the United States deems Iran an enemy. Note that Iran’s government’s viewpoint is not necessarily the same as those of the Iranian people. Additionally, the state of Iran funds many proxy terrorist groups across the Middle East, all of which do terrible terrorist things, which the U.S. doesn’t like at all (especially when they do terrorist things against American citizens living in the Middle East).

Israel and Iran’s rivalry (and you can include the Saudi Arabia, UAE in this too) is mostly down to wanting to be top dog in the Middle East, having the most influence and power, and it doesn’t help that they have very different beliefs (not just religion, different styles of government, different policies, that sort)

TLDR:

U.S. supports Israel for Middle East influence

U.S. don’t like that Iran’s government is anti-America and anti-Israel, and they don’t like that they support terrorist groups either

Israel competes with Iran for power in the Middle East, and have very different beliefs

Please note that I am not trying to incite a political debate. America and Israel has done bad stuff (war crimes, imperialism, etc), Iran has done bad stuff (terrorist proxies, political repression, etc), both are bad in my book.