You're asking two different questions.
With regard to the first, why should you care about Israel, Israel is important because Israel is the US's most important ally in the Middle East, a rare outpost of democracy and modernity in a backward, anti-American and despotic region.
With regard to the second question, shouldn't you be more worried about the interests of the United States? Absolutely. And I don't think you could argue that any American politician from any party doesn't pay attention first to America's interests. Nobody's arguing that they shouldn't. It is important to note, however, that America's interests and Israel's interests often coincide. This nut Achmaedinejad, who cries "Death to America" and "Death to Israel" in the same breath, is a perfect example of that.
Isn’t it possible that they’re our ally largely because we always side with them?