To add to the previous answer, which did hit upon main points, it is important to point out the conditions that existed in Germany in the period between the world wars. And, the question cannot be answered without reference to the political thought of Adolph Hitler as expressed quite explicitly in the book he wrote during his time in prison for attempting to overthrow the government.
The causes of World War I were many, and numerous countries were at fault for its outbreak. To the extent that the victors of a war dictate its aftermath, and that was certainly the case in 1918, then Britain and France helped shape the circumstances that lead to the rise of the National Socialist Party (the Nazis) in Germany and to the ability of Hitler and his followers to launch Germany on a path to war. The conditions set forth in the Treaty of Versaille were designed to keep Germany in a subordinate condition relative to the victors, and included strict limits on Germany's ability to rearm.
Combine the sense of national humiliation that Germans felt living under the restraints of the treaty with the onset of a global depression and the conditions were ripe for the rise of the Nazis. Exploiting high levels of unemployment and the economic burden of reparations payments to the allies under the terms of the treaty, Hitler, a gifted orator, appealed to the basest instincts of a defeated people and pointed the finger for all their problems on "world Jewry." Not only did Hitler lay the blame for Germany's troubles on Jews, but he argued to a receptive audience that Jews had been the cause of the World War I in the first place.
Jews had lived for centuries in Germany and had risen to levels of responsibility in many professions, including the military. The misery of the German people in the interwar years, with the depression creating massive unemployment, left them vulnerable to the propaganda machine run by Joseph Goebbels, Hitler's Reich Minister of Propaganda. Playing upon broad perceptions of Jewish influence in the banking and financial industries, the emotions of millions of Germans were stirred up by Hitler's passionate denunciations of Jews and of the unrelenting propaganda produced by Goebbel's ministry.
Preconceptions or prejudices about Jews that existed across Europe made it easy for the Nazis to appeal to the worst in people desperate for change. That Hitler and many of his followers genuinely believed in the hatred they spewed is undeniable. Hitler's book, "Mein Kampf," is very pointed in his determination to rid Germany and Europe of Jews if he should ever have the opportunity to do so.
It is difficult to know this for sure. We cannot really know what causes a group of people to hate another group. We are left to try to make educated guesses or conjectures.
One cause of Nazi anti-Semitism was the deep-seated religious feeling among many Christians that the Jews were responsible for the killing of Jesus Christ. This feeling had helped to fuel anti-Jewish feelings and actions at least since the Middle Ages. We know, for example, that Jews were sometimes blamed for the Black Death. Thus, at least some degree of anti-Semitism was latent in much of the European population by the time that the Nazi Party arose.
The Nazi ideology needed a scapegoat. If the Germans were, as Nazi ideology insisted, the master race, it was necessary to explain why they had not been able to dominate the world up to that point. They especially needed someone to blame for the problems Germany was experiencing after WWI. Therefore, they needed someone to blame. Jews seemed to fit the bill for at least two reasons. First, people already had some tendency to dislike them. Second, they did not fit into the Nazi ideas of nationalism since there were Jews scattered across Europe.
Thus, the Nazis were looking for scapegoats and the Jews were convenient. This led to them conceiving a true hatred (stoked and exacerbated by relentless propaganda) for the Jews.
Even though Hitler was a jew he hated jews. sometimes I think that well if he is a jew himself then why does he kill them. well here is what I personally think it is. he hates his mom so he killed all jews thinking that they were bad people.
The Jews were one of the largest minority groups in Germany that were not only financially successful, but also disproportionately well-represented. They remained largely a separate community, refusing to assimilate but still retaining much influence in society due to their wealth and standing. Anti-Semitic sentiments, which were already present in German society, was thus taken to a higher level. The Nazis, under Hitler, consistently viewed the Jews as a cosmopolitan diaspora that were the parasites of society and a potent force that refused to integrate into the societies they lived in. They believed that the Jews would in turn manipulate governments into waging war against Germany since it was in their interests to do so. Thus, they saw it as necessary that Jews be made to pay the price by destroying them through whatever means possible.
This answer is factually incorrect in a key area. Germany's Jews were, in fact, fully assimilated into the country's society, which was one of the reasons so many of Germany's Jewish population were unable to grasp the severity of their situation until it was too late. This is not a minor detail, and I hope no students draw the wrong conclusion from reading this answer.