I have been trying to investigate the roots of anti-semitism all my life. I can understand some amount of anti-semitism that does not exceed "normal" xenophobia. To some degree, xenophobia is "normal," insofar as it is consistent with the group survival mentality. If you want to promote the survival of your group as a unique cultural and linguistic entity, some amount of resistance to other groups is inevitable.
However, I have discovered, that anti-semitism cannot be explained by xenophobia alone. It cannot either be explained by economic competition alone. It is much, much greater than this. Anti-semitism is an irrational, overwhelming hatred of Jews that cannot be put down to any rational or historical explanation. Anti-semitism is rife in countries that have never had any Jewish population. It is a pathological and hysterical state of mind that has one meta-explanation for everything that is wrong with the world - the Jews. Ultimately, it is a disease of the spirit. It is also satanic in its origins. The irrationality and exaggerated nature of anti-semitism is to me one of the proofs of the existence of G-d.