Being Unsentimental is the vice of deficit for the virtue of Honor, or "venerability," which is an objective curiosity about the past and its value to us today. Someone who is unsentimental will tend to dishonor the past as if it doesn't matter. They may be "revisionist" about history, both global, or specifically the history of the relationship, and any harm done in the past by one partner to another.
Like any virtuous psychological trait that falls on the Golden Mean, there is also a correseponding set of two vices - one of excess, which is Nostalgia, and one of deficit, which is the state of being Unsentimental. Instead of agreeing with everything the other says, they both need to have the honor to help each other correct their own, faulty beliefs, through a sense of curiosity about the results of the past, not hostility or resentfulness about it.