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Historically, I think it stems from those who were threatened by dual nationalism, and an adherance to a code of law beyond whatever the ruling powers dictated. Being the oldest, or first brings the type of animosity you don't hear much of from say, the Kurds, or Druze. Christian and Islamic scriptures have some issues built into their text by nature of adopting the Torah attempting to be a continuation.

The rest comes from inferiority complexes and a jealousy for what Jews have accomplished. I lot of people don't like to hear that a minority group which makes up 0.25% of the world population, or less then 2.0% of the US population has been so accomplished and influential. That pisses people off. I'm sure people reading that right now are irked and want me to shut the fuck up just for mentioning it.

The thing about asking why is, it doesn't really matter why. Asking why makes it sound like there's a reason. Really irrational bigotry doesn't have any reasoning.