Apparently his mother was very sick and her doctor was Jew, and he blamed the Jewish doctor for not saving her.
Secondly, after WWI Germany were paying back heavy reparation because of the Treaty of Versailles, the economy was failing terribly, none of the Germans had job, the only ones who seemed unaffected were the Jews. This led Hitler to believe that the Jews were the reason for all of economic suffering in Germany, Very stupid, but yeah.