When we feel guilty

Why do we?
For example my ex told me he really never suffers from guilt. No surprise haha. I am starting to feel guilty over the email I sent him, remembering how he said I was all he had, and he loved me more than anyone else, even thou he wasn't "in love" with me. I spent all my days encouraging him, loving him, giving to him, only to be left in the cold with ST many times.

So I wonder, why do I feel guilty? (For those who don't know I basically said fuck you, I don't want you in my life nor need you there." And blocked him) I never say things like that to anyone, even if they're my worst enemy. I usually just ignore them.

I know I shouldn't feel guilty at all. He chose to give me ST after hurting me yet again and then acting surprised when I cried. But I still feel it. Does anyone have experiences with this and answers to why we feel guilty about crap like this?