Because when a person you amor lets you down, betrays you or does something unspeakable, it is a thousand times worse than if any old stranger did it because you trusted them. And when that trust is chucked back in your face, the worst kind of hatred is born. Emotional damage is some serious stuff :|
I don't per say despise them but I know I don't like who my ex became, it can happen when someone you amor hurts you, emotionally or physically or both which both was the case with my ex, he was my first amor & we were married for 23 years but after going through that it changes the way you feel about someone.
It depends mais on the individual situation. I'm not one to believe that you can truly hate something or someone that you once truly loved and vice versa. However, this is me and I have issues with attachment.