I'm afraid I just have a compulsion to not waste anything (except possibly words, certainly parentheses).
My obsession with turning off taps comes mostly from the horrible noise the cold bathroom tap made in the house I grew up, when my big brother would turn it on and not turn it off, ever, so I could hear it in my room and have to turn it off.
There may be no very good reason to save water (but everything takes energy so I suspect there is) but there is no good reason to waste it, and so I automatically choose not to. It's less effort.