Who Deserves a Second Chance?

So, anyway.......
How do you know if an ex deserves a second chance? I know a lot of people who have gone onto marry a person with whom they had at least one break-up. I know it can work, I just don't know what the determining factors should be.  I've never really been one to go back to an ex.  I'm cold and heartless, I suppose.....  Once I'm done, I'm done.  But, lately I've been a little curious about a guy from my past. We didn't have a bad breakup, things just didn't work out. We're both older, and presumably wiser, but I'm still weary. I've never left a relationship thinking that I could have done more.  I try hard, because I don't want to have that, "he's the one that got away", moment. Well, I don't want that moment to come because I could have tried harder. Basically by the time a relationship has completely run its course with me, I know I've done all I can, or am willing to do to save it.  Anyway, feel free to email me with your thoughts.