Like the expression goes, because your Ex is “the devil you know vs. the devil you don’t know.” People who are bored or very much in need of an “intimate” interaction, find it much easier to go “backwards” in time to try to hook up with someone who they already know, than to go “forward” in time to find a new and hopefully more compatible partner. Going back to an ex eliminates the effort required to meet someone new and allows you to cut the time invested to get to the point of a date or sex. You already know your ex so it is easier and seemingly safer (though not really), to just pick up the phone and have a date the same night, rather than go out and try to pick up someone new. Women, in particular, when single and in need of an “intimate” interaction, feel much better about sleeping with someone who they have already slept with in the past instead of adding another person to their roster. Some people think that it would be disappointing to add someone new to your list only for them to be horrible in bed when they think that their ex can handle the job (and screw up the rest of your life!)?
Also, because some people really haven’t moved on, they still believe that their Ex is the best or only person for them. They still don’t see the potential out in the world to meet someone who is a better match so they convince themselves that they have to go back, otherwise they have nowhere else to go.
Let’s be clear about something, there are six BILLION people on the planet. Regardless of how caught up you feel on one person, there is always someone else out there for you to meet and fall in love with. Don’t ever limit your own growth and happiness by thinking that the last person you dated is your ONE and ONLY true love.