Let's first start by saying that anti-semitism existed in Western Europe prior to the birth of Hitler, with pogroms against Jews occurring long before Kristallnacht. Jews, as well as other minorities, had long been the established scapegoats. Since the Middle Ages, Jews were forced by European Christians to work & reside in ghettos, preventing assimilation. Only certain occupations were open to Jews, such as banking & moneylending; trades which made some Jews, like the Rothschilds, wealthy but also the targets of envy and hate.
The scapegoating of European Jews was often politically and religiously expedient to those in power, such as the Catholic Church, and those seeking power. Martin Luther, the German founder of Protestantism, was a vehement anti-semite who wrote vicious rhetoric about the Jews and worked toward their expulsion from many German towns as early as 1536.
Adolf Stoecker had formed the Christian Social Workers' party in 1879 and by 1883 riots, as well as the burning of a synagogue in Neustettin, were stoked by Stoecker's call for a "final victory" against the Jews. He was followed by Professor Eugen Euhring, speaking at Berlin University, of the "obligation to drive an inferior race from public honor", and Gottingen University's Paul de Legarde's (aka Boetticher) suggestion of a three-fold program: the Germanizing of Austria, the conquest of Russia, and the expulsion of Jews to Palestine.
This would have been the zeitgeist in which Hitler was born.
During his youth, Hitler found himself in Vienna struggling to become an artist and attempted to pass the entrance examinations at the Academy of Fine Arts. He failed. At this point, Hitler became a drifter and struggled to survive from time to time as a day laborer. In Vienna, Hitler would have encountered a thriving Jewish community, where Jews held distinguished positions in universities, such as the one that just rejected him, and private financial institutions. He also would've encountered open anti-semitism in print and on the street. It is speculated that Hitler blamed all of his personal failures on the Jews, despite the fact that it was a Hungarian Jew by the name of Josef Neumann who once saved him from starvation.
Hitler's personal anti-semitism grew as his public political power and megalomania grew. One of his greatest political enemies were the Communists. Although Jews made up the ranks of every political group, Jewish participation in contemporary left wing politics of the early 20th century was established and formidable, going back to Karl Marx. Hitler's sad theories of a "master race" conveniently tied into the annihilation of his political opponents as well.
And yet, Hitler wasn't unaware of the absurdity of his "master race theory". He admits as much when he was quoted as saying "I know perfectly well that in the scientific sense there is no such thing as race. But you, as a farmer, cannot get your breeding right without a conception of race. And I, as a politician, need a conception which permits the order than has formerly existed on a historic footing to be abolished."
Again, these are just speculations about why - or how - it was possible that Hitler hated the Jews. It's also likely that he was a certifiable sociopath, in which case no explanation is necessary.