All memory is acquired in the context of a particular emotional state. Most people remember where he was and what he was doing at the time of the accident that resulted in the death of someone important, but no one remembers the face of the person who sold him the last movie ticket.
The emotional impact of someone important news is big; The memories recorded at that time were influenced by this intense emotion. The timing of buying a movie ticket can often be emotionally insignificant. The emotional importance of each event directly interferes with the acquisition of new memories.
The hippocampus selects what has value to remember or not. We know this from a very famous case in neuroscience, where a 27-year-old had an injury in this area. He remembered who he was and what had happened in the past, but from that time on he could not store new memories. This shows that the hippocampus plays a very important role in memory storage. And since it is tied to emotion, we believe this explains why the brain stores some things and not others, according to their value to us.