A couple of days ago I wrote about how I like to read and learn new things. So much that I just can't remember all of it. And even that little which I do probably replaces something I previously used to know.
Except, as I have read, that's not how our memory works. All the things are still mostly there. They remain in our heads indefinitely, at least almost. It's just that while the amount of information grows some of it becomes less relevant and is therefore buried beneath the more important information.
And no, there isn't any sense on what is deemed more important by our brain. It's simple logic: the most often used information is the one that gets the priority. The things we repeat over and over again.
But if so, I have no idea why I do remember some obscure and trivial things from a few decades back while I can't remember some more recent, and more important things. Probably due to the final factor to the equation: interest. If the information is interesting it seems to stick better with us. Naturally, if it's interesting it gets repeated more often leading back to the most repeated info gets the priority -thesis.