Pain

The most scary sound i can ever imagine is a dentist drill going on and on in my ear, even the thought scares the shit out of me. After one of those rare visits that i actually grant my dentist to have the pleasure of being with me. I had a rather unlikely and unpleasing doubt in my mind, with my gum's aching like sour wounds and the rusty taste of blood making it difficult for me to swallow the intelligent side of me asked the doc a question- Why is it that people always refer pain with a phrase like 'Pain in the ass/ arse' , why not relate it to the most difficult and obvious one like 'Pain the tooth'.