As I have observed some memories become more prominent based on two main aspects, the intensity of the emotions at the moment of the event, how unique is that memory compared to other different events and the feedback we receive along of the time by the other people who also shared the memory.
By the first aspect as stronger the emotion was at the moment of the event as easier to have a more lasting imprint it in our memory.
The second aspects relate on how unique was the event on our memory, the more our brain becomes accustomed to the associated feelings, it diminish the significance of that memory over time.
And lastly, it is important to take into consideration that our memories are constantly shaped into new ones according on the feedback we are constantly receiving by other people on how other people lived and remember the same event.