Another one of many idiosyncrasies of the human race. We are one of the only species to wash ourselves to get clean, much less actually holler our horribly out of tune choruses of random squawkish rabble otherwise known as singing. But it is a rather interesting, foible is it not?
But why do we sing in the shower? Now I know there are many people who hum tunes to themselves when they find themselves alone with no one around, but is it about being naked with water running over our bodies that makes us wish to constantly burst into joyous song? Is getting clean that much fun? Maybe it's some kind of chemical reaction that takes place when were are exposed to steamy water. Alas, 'tis like how many licks it takes to get to the center of a tootsie pop: the world may never know.