A quote from Rabbi Hershel Schachter:
Hashem selected one nation to serve as a role model for all the others regarding how to go in His ways. The novi Yeshaya says, “am zu yotzarti“, i.e. that the am ha’nivchar was a distinct creation of Hashem. According to nature, Klal Yisroel really should not exist, since the Chumash tells us that the imahos were akoros (unable to bear children), and the Talmud tells us (in Yevomos perek He’oreil) that Avrohom, Yitzchok, and Yaakov were akorim as well. As such, the whole existence of Klal Yisroel is l’maaleh min hatevah.
This probably explains the mysterious phenomenon of anti-Semitism which persists throughout all generations. A body naturally rejects foreign objects (and therefore when surgeons do an organ transplant they have to be concerned about the organ being rejected), and Klal Yisroel does not fit in to the natural system which makes up the rest of the world; Klal Yisroel was created as a separate yitzira which is l’maaleh min ha’teva. Thus we can understand quite well why all of the nations of the world, which are all part of teva, would naturally reject the “foreign body” of Klal Yisroel which does not fit in with the natural scheme of things!
See the rest here.