I believe it was because when he was 18, his mother had died. They were very close and he always carried a photo of his mother around with him. He was convinced that a Jewish doctor had been responsible for the death of his mother.
Another reason is that whatever happened, Hitler and the Nazis said the Jews were responsible for events such as World War One and the economic crisis.
Hitler believed that the Aryan Race was the strongest and best race. He also believed that Jews were the inferior race and that they were not to be considered as people.