Start reading a journal you wrote a decade or two ago, and within a few pages, you'll find ideas you could have sworn you only came up with in the last few days or weeks, albeit slightly differently formulated.
You dig yourself into a rut. It becomes your world, a riverbed that your thoughts rarely overflow.
Didn't Ezra Pound say somewhere that, as we get older, we discover more and more how true were the things we believed when we were young? This probably has to do with selective perception. Darwin wrote in his autobiography that he found it necessary to write down every piece of evidence which appeared to contradict his beliefs, since otherwise he knew they would disappear from his mind.
“As I'm sure you know, water always picks the shortest route to flow down. Sometimes, though, the shortest route is actually formed by the water. The human thought process is a lot like that.” -- Haruki Murakami, “Where I’m Likely To Find It.”