Why would someone do this?

My so broke up with me out of the blue one day for no reason, we started yelling and I told him " I wish you gave me a reason to hate you". Next day, we talk again and he told me he had sex with an ex. Time goes by and we start talking again, he apologizes and regrets everything. He then proceeds to tell me that he lied about having sex with her, they just talked and kissed. Reason for lying was he felt very guilty and wanted to give me a reason to hate him. He didn't think I would be able to look past what happened. Can I get someones opinion on this situation. He now says he wants to be with me and something like this will never happen again.
Why would someone do this?
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