Nostalgia is the vice of excess for the virtue of Honor, or "venerability," which is an objective curiosity about the past and its value to us today. Someone who is too nostalgic might be sweet and charming about good times shared, but in "living in the past," they may also be "out of touch" with the state of the relationship currently, and may make errors about their career, believing that it is currently in a more valuable state than it really is, which will harm the couple's finances.
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.