To dream about your ex-boyfriend/girlfriend or ex-husband/wife or that you and your ex got back together again, suggests that something or someone in your current life that is bringing out similar feelings you felt during the relationship with your ex. The dream may be a way of alerting you to the same or similar behavior in a current relationship. What you learn from that previous relationship, may need to be applied to the present one so that you do no repeat the same mistake. Alternatively, past lovers often highlight the positive experiences you had with that person.
In particular, to see your ex-husband/wife in your dream, indicates that you are finding yourself in a situation that you do not want to be in. It suggests that you are experiencing a similar relationship or situation which makes you feel unhappy and uncomfortable.
To see your mate's ex in your dream, suggests that you may be comparing yourself to the ex. The dream is trying to tell you not to make the same relationship mistakes that ended that relationship.
Alternatively, seeing your ex in your dream also signifies aspects of yourself that you have x'd out or neglected.