To dream about your ex-boyfriend/girlfriend or that you and your ex got back together indicates that people currently in your life are reminding you of those same feelings. This dream could be warning you that you are falling into a repeated pattern in relationships. You should consider the harsh lessons you may have learned so you don't repeat them. Sometimes, however, past encounters can be satisfying episodes in your life. This dream may also indicate things you have X'ed out within yourself.
To dream about your lover's ex could be that you are trying to live up to their standards. This dream is warning you not to do the same thing you did in a previous relationship.
To dream that you are kissing your ex suggests that there are certain attributes about your ex that you enjoyed. This dream isn't indicating necessarily that you wish to visit the relationship again, but that you should think about what your ex's better qualities or attributes. These are the attributes you are finally coming to acknowledge within yourself.
To dream that your ex has died suggests that the emotions you had for your ex are finally over. You have moved on and he or she is now relegated to your past.