One really good reason I can think of is if the break up was simply due to bad timing of the initial relationship.
You were at different stages of growth when you were together, and your goals and dreams at that time didn’t mesh. Maybe you were still in grad school, trying to figure out what to do with your professional life, and that ended up taking you to another city.
Maybe neither of you were mature enough to really settle down.
Maybe one of you had just broken up or divorced and needed some solo time to regroup and got into the relationship too hastily.
… or maybe, after time spent apart, you both realized that the two of you were a couple of fools, and there really is no one better for you than each other. This is a really good reason.
You know, I’ve heard astounding stories of exes who come back together again, even after many, many years. Sometimes they were college sweethearts. Sometimes they used to be married. If the window to your respective hearts is still open to each other—if all is forgiven—I don’t see any reason not to make another go of it.