I am sure there are many reasons for this, but I suspect that, for many exes who express these sentiments, it's a different version of the same old thing - "the grass is always greener." Throw in the fact that it's pretty damn safe to pine after someone who is no longer available (no push for a commitment there) and voila! I wonder how many of these exes would come running if "the one that tragically got away" suddenly became available?