Here is the only reason an ex will ever come back that I've seen in my profession. The late great Thich Nhat Hanh said, “You must love a person in such a way, that the person that you love, feels free.” What that means for me is, when a person feels like they can be themselves, when a person feels that they can come and go, if they please, they have no obligation to be with you. This is the pressure that's let off of them when they break up with you, because the attraction was dwindling, or they felt like this wasn't the relationship for them.
When they come to realize that that's not what they want, it doesn't feel like something that's comfortable for them, that's when they want to reach back out, and that's when they feel like they can give the relationship another chance.