Relationship Warfare: The Exes of Evil

Although few of my relationships have been anything to write home about, I usually end up simmering in them months after we stopped liking each other until things end on extraordinarily bad and awkward terms... call it an exit strategy.

For some reason I've tried to stay friends with every one of my exes. This has never resulted in a positive situation.
Negative effects from these friendship attempts have run the gamut from being called a soul sucking succubus (nice alliteration, yes) to having the new psychotic girlfriend of the old psychotic ex regularly stand outside my house crying and throwing snowballs at my window.

Why is it so hard to let go?