The mathematician Poincare has a great essay he writes about this kind of thing. Roughly, the creative process happens in two stages, one where you submerge yourself in the workings of whatever you’re learning about, and another where you have to relax and let inspiration come.
Essentially, the unconscious mind is the source of the great insight (which Poincare argues it arrives at by chance, at some level), but you have to allow your conscious mind to relax to let it through. Poincare had one of his great ideas when he was stepping on a bus. Einstein reportedly would doze off with objects in his hands that would fall and wake him up, so he could write down whatever was in his mind at the moment of falling asleep. I think the shower fits the bill as a perfect place to relax and let whatever is percolating in the mind come to you.
Central here though is a response to our assumption that we can think our way to good ideas. In fact, we can only think our way to prepare our mind to have good ideas. Then we have to relax. You can’t force it.