A few reasons. The first is the sound of the running water. It's white noise that blocks out the sounds of other things in the environment. Then we have water on your body at a controlled temperature. These two things together limit the amount of sensory input from your skin and ears. And then, we have our eyes. In most cases they are closed or not looking at anything in particular. Now we've eliminated one more form of sensory input. So, essentially, a shower is a poor man's sensory deprivation chamber.

The natural effect of this is that you are alone with your thoughts. Without sensory input, your mind will naturally wander and begin looking within for stimulation. This usually means processing existing information and creating new things to keep it occupied.

Source: http://www.quora.com/Why-do-ideas-come-to-me-so-easily-when-I-am-taking-a-shower/answer/Terry-Prothero


I feel strongly to share this as I relate to this quite a lot and it's always been a mystery to me and I know there are several other people experience the same.
When ever I'm have a tough nut to crack whether in my area of specialization(programming) or in real life activities, most times whenever I take a shower around such periods, ideas just keep pouring into my mind kinda unconciously.

You're not alone. I get a lot of ideas when I'm in the toilet.

tocineworld: You're not alone. I get a lot of ideas when I'm in the toilet.

Similar but at the same time different though.