There’s probably as many theories as there are people.

My own is that cognitive dissonance is a powerful force, especially for a narcissist. Hitler spent 4 of the best years of his life is the pure misery of the trenches of WWI, and even worse - running between the trenches to deliver messages. He was gassed so badly it put him in the hospital for the final month of the war. Hitler was probably also deluded about Germany’s failing position in the war, probably kept in the dark by German propaganda and made quite gullible by his fierce patriotism. So when Germany surrendered, that must have actually both shocking and enraging for him. And we know that because he frequently attacked the treaty of Versailles.

The “stab in the back” myth is the idea that Germany’s surrender was corruptly carried out by politicians, and due to common antisemitism across Germany and Europe generally it soon focused on Jews. In reality there was virtually no Jewish people significantly involved with the surrender and events leading up to it, but that didn’t matter because as is often the case today, emotional truth is more important than reality. Given that Hitler couldn’t possibly bring himself to believe that he incurred tremendous pain and suffering by voluntarily and stupidly buying into an imperialistic war that was doomed to failure from the start, the stab in the back myth would’ve been extremely appealing to him. He wanted to believe it, so he did.

So when he happened to be assigned to monitor the DAP - the predecessor to the Nazi Party - he was ready to wholeheartedly accept their antisemitic ideas. Even though he seemed to get along with Jewish officers during the war, by a year after the war he was already talking about “removing” the Jews.