Google isn't helping
Hi there,
This question comes up a lot, and ultimately the premise upon which it is based - that the Holocaust or Hitler's hatred of Jews stemmed from some personal event or experience - is not supported by evidence. Anti-semitism in German and Austrian culture during Hitler's life was was pervasive and helped shape the man and his narrative of racial prejudice.
The following answers go into detail in addressing your question and its premise, and will hopefully be of interest to you:
We don't know.
He did write in Mein Kampf that he never had a problem with Jews until he moved to the city and saw Orthodox Jews, which he described in all sorts of vile ways. Most Jews were assimilated (ie it was not apparent they were Jewish). Orthodox Jews were not. They maintained a distinct culture.
Antisemitism was extraordinarily common and popular, so "becoming" anti-Semitic was in no way unusual or unique. It's part of the reason people didn't believe he would engage in mass murder-antisemetic rhetoric was common in politics and he didn't seem remarkable or unique in that regard.
On a side note, when Hitler's mother had breast cancer, a Jewish Doctor (Dr. Bloch) provided her care at a greatly reduced rate and Hitler was always thankful. He wrote letters to the doctor for years and gave him a protected status after taking power, although eventually Dr. Bloch was forced to flee, although he was allowed to take more with him than other Jews and was shielded from harassment, arrest, and death.