Generally speaking (without knowing the particular details of a break up), I will list only some of the reasons that would make someone want their ex back:
- the belief you are still in love with them (sometimes it can be true, while others it is an ilusion; people are in love with the feeling of the happy memories and that's what they miss);
-the fear of being alone (emotionally speaking), never finding someone new to love and be loved by;
-the tendency to idolize them, to intensify the goods they had and forget the misery they put you through;
-the fear of starting all over again, from scratch;
Every time people break up, especially the ones who have been left, feel extreme pain and sense of rejection and that hurts so bad, so just for the validation's sake (that they are worthy of love, and deserve it), they want to get back even in unhealthy relationships to "prove" to themselves and the "loved" one they are WORTHY!
Is it worthy??? You can't generalize, everything depends on the story behind the relationship and the people involved. In my opinion, time is the best judge.