John Cleese remained close friends with his first wife after their divorce.
My assumption is that the hater is still in love with the ex, but the ex moved on and the hater is angry at her/him for it.
If the hater was to accept the ending of the relationship, he would have stopped caring about the ex, but he still does, by hating. Hating's still a way of caring.
Not always like this, but most of the time, IMO.