Why did the US support the Shah of Iran?

It is worth noting that Iran has had hundreds of Shahs, most living before the US even existed as country. As a result, most were not supported by the US.

Assuming this question is referring specifically to the last Shah of Iran, Mohammed Reza Shah Pahlavi, the US supported him because he was amenable to US business interests and allowed them easy access to oil. More importantly (politically), he was neither communist nor Islamic fundamentalist and did not represent a threat to neighboring countries in the Middle East.