I don't invest my free time with exes. Therefore it is unlikely to ever happen. The way I view relationships is you meet someone who has the traits you're looking for and you put your all into the relationship.
If it doesn't work out there is nothing you could have given that you didn't already give! Unless two people become two (different individuals) there's very little reason to repeat a lesson you've should have learned by now.
After breaking up it's very common for people to "romanticize the past" with rose tinted glasses. They remember the "good times" and forget what led to their breaking up. This is especially true if they are (lonely) and haven't found someone else to date or connect with.
In order for your (ex) to be "the one" they would have to see (you) as being "the one". At the very least a (soul mate) is someone who (actually wants to be) with you! (and vice versa)
If there was a breakup one of you felt you could do better!