Because we don't know ourselves.
And because we are unaware of the fact that the other, in this case our partner, acts only as a mirror to ourselve.
And also, because we, unawarely, projected so much in this person that we seem incapable of letting go.
We think we loose a part of ourselve, which of course is illusory.
If we would understand this, we wouldn't feel the desire to take revenge.