1. They miss physical intimacy
That one is pretty obvious. If at one point this was easily attainable in a relationship and not so anymore, then sex is a big one to come back for.
2. They miss having someone there
Does your ex hate eating alone? coming home to an empty house? Then most likely they just like having you around...
3. They miss you as a person
If your ex makes a point of telling you how much they miss you, and all the details of what they miss about you.. then chances are- things may work out!