First of all; a bit of context. There’s a collection of structures in the brain called the Limbic System that combine very effectively to keep us alive. You could say it’s the survivalist part of the brain. One of these structures is the amygdala. It’s primarily in charge of the Fight, Flight and Freeze response. But also in deciding which memories should be kept fresh and present, and which memories can be filed away. The amygdala works closely with the hippocampus a structure with an important function. It forms new memories and links emotions, sounds, smells and other relevant sensations to these new memories.
And because the Limbic System is there for our survival one of its characteristics is its quick reaction time. Its nimbleness and its somewhat scattergun approach to what it deems important. And because it has to react quickly there’s no time for constant, fresh interpretations. Instead, it patterns-matches; hence the memories it chooses to keep fresh for our survival.