“Perfection is the death of all good things, perfection is the death of pleasure, it’s the death of productivity, it’s the death of efficiency, it’s the death of joy. Perfection is just a bludgeon that goes around murdering everything good.”
So under conditions of stress and fear we tend to focus on the negative — particularly the negative in our partners! This feeds the negative cycles. Fear also narrows our perceptions so that we tend to stereotype the aspects of our partners that we don’t understand or don’t like (which often are the same things).