Our lives move fast cluttered as they are with details, chores, small rituals. We barely have time to collect a few memories (the image I have is of orchids placed selectively in a hothouse.) We keep going back to them and so they become stronger in our mind, like a recited chant.
There is an old philosophical puzzle. If you replace a plank on an antique ship, it is still an antique ship. But, if you replace all the planks, you have a whole new ship. At what point, the puzzle asks, does the antique ship become a new ship?
We ravage our memories like this too. Each time we visit them we re-play the story in our mind in a subtly altered way. We smooth out the narrative. We omit awkward or incongruous details. Soon, our own re-telling has more weight than the original. We've lost our first snapshot and all we have is the memory of those memories, a spiraling well of revision.