It is fallacious to assume that Hitler's anti-Semitism was unique or even new. Jews throughout Europe had been vilified since the Middle Ages because they were (1) Not Christian, (2) persisted in retaining their traditional religion, culture and values, and (3) were the money lenders at a time when that practice was denied to Christians. Jews in England were slaughtered immediately after the coronation of Richard Lion Heart. Similarly, before the First Crusade was launched, Peter the Hermet's Peasant's Crusade killed thousands of Jews in continental Europe.
Although most Europeans by Hitler's time had learned to live peacefully with their Jewish neighbors, an anti-Semitic element was always present. As a young man, Hitler was an avid reader of an anti-Semitic (and at times pornographic) periodical known as Ostara. He was also enthralled with the music of Richard Wagner, who was a virulent anti-Semitic. Because many of the early Communists were Jews (Marx, Trotsky, Rosa Luxembourg) Hitler easily associated Communism with Judaism. The threat of a communist revolution in Germany following World War I lent itself to the connection in Hitler's mind.
Hitler's anti-Semitism was thus an element of his culture. This alone does not explain his attempts to exterminate them, however. All National Socialists were anti-Semitic and pro-Nordic; Hitler's gift for oratory and his personal charisma brought him to the forefront of that movement and he thus became the figurehead for the anti-Semitism that was already present.
Hitler had strong ideas about racial purity and the superiority of the Nordic race. He hated the idea of Jews and Germans intermarrying and produciing children who were half Jewish and half German. He probably would have been glad to drive all the Jews out of Germany rather than trying to exterminate them, but this was impossible because they had nowhere to go. His obsession with creating a pure Aryan master race which would dominate the world was part of his grandiose and megalomaniacal plan to shape human history. It caused horrible suffering for millions of people. Ultimately even his loyal supporters recognized that he was a madman with absolute power, somebody comparable to the Roman emperor Caligula.
It is impossible to know why any one person comes to be racist or anti-Semitic. We can have ideas as to why many people in Hitler's time and place hated Jews, but we cannot know for sure what motivated any individual.
We believe that Hitler and others like him hated Jews because they were so different. Jews had kept to themselves in places like Germany for a long time. This was partly because they had been hated and discriminated against for years. But their separateness made it more possible for people to hate them. They were seen as a group of people who were fundamentally different from those who lived around them. Because of this, when people looked for scapegoats, the Jews were the first choice. Because people had long looked at Jews as an alien presence who could be blamed for anything bad that happened, Hitler and many others like him chose to hate them.