Some of what memories we keep is intentional.
I heard a neurologist make a point that was very revealing about how the brain works --- that a memory in the brain does not work like a videotape, rather it's a story one tells oneself, over and over.
In the process of that reteling of course, things get embellished. And other parts get eliminated. Or merged with other stories.
I always thought of my oldest memories, like the day my sister was born, as more like "memories of memories".
Thank Pogo, I would always think that it worked like a videotape and that we remembered a memory maybe because at that moment something different would happen, feeling, scent, sensation ?
Yes certainly there are associations, some of the time. Something in the right brain -- a piece of music, a vision of a landscape, and so forth. These can trigger what the memory was, but I don't think there is always such an association.
I can remember vividly the voice of our high school principal on the building's public address system telling us that President Kennedy was dead. I remember where I was sitting, what the view outside the window was, what the sky looked like, how our teacher reacted, all of that. When something meaningful happens we go into a heightened awareness. Other times it's much more subtle, that the brain just happened to receive two different stimuli at the same time and desired to remember one of them, and the other 'hitched a ride'.
But yes, a story we tell ourselves, over and over until it becomes a belief. If we listen to ourselves carefully we can start to hear how the story changes over years. Then we start to have doubts about what really happened.