Why did he actually breakup with me?

I recently got broken up with. He told me that there was something in the back of his mind that kept nagging at him that something wasn’t right in the relationship. He told me the sex was the good, that he thinks I’m a beautiful woman, but that in the back of his mind something kept nagging at him. I am so lost, it is crushing my soul.
Why did he actually breakup with me?
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