Your ex may not have anyone else to turn to for comfort or companionship.
They may feel guilty about the way things ended and want to make it up to you.
You represent a source of comfort and familiarity that they miss.
If they relied on you for financial support, they may come back when they need help again.
Your ex may have a fear of being alone and may come back to you because of it.
If they see you with someone else, they may feel jealous and want to win you back.
It may simply be more convenient for them to come back to you than to start over with someone new.
Your ex may be remembering the good times and want to relive them with you.
They may have unresolved feelings for you and want to see if there's a chance to rekindle the relationship.