The brain is always full. For a new memory to form, an old one must be forgotten.
You never know which memory was sacrificed because that's the nature of forgetting.
'Liquid' - 2018
Are cities like human memories?
There's not enough room for all the buildings, so if we need a new one, we have to knock down an old one.
Every time you go by a new building, your awareness of it gets stronger and your memory of what was there before gets weaker.
'Things Forgotten' - 2018
If you want to remember the way something used to be, you must never go there now.