- they choose their mates randomly or based on superficial things (so there is no need to stay in touch after a breakup),
- they are horrible people (so the other side does not want to stay in touch),
- they keep finding horrible people,
- they need a scapegoat or are vengeful,
- they are overly emotional and cannot get over a breakup,
- they are overly envious.
(I am in good relations with most of my ex-girlfriends.)