Root Canal

I went to the dentist this morning (he isn't as good-looking as the cartoon dentist) to get my teeth cleaned now that we have dental insurance again, and I found out that lucky me gets a root canal. My teeth have not been bothering me at all. He said I must've knocked it when I was like 12 and it has been dead ever since. What I want to know is why the heck hasn't anyone else noticed it? When I lived at home, I went to the dentist every six months, and in the last 5 years I've been to about 5 different dentists, and no one has ever noticed that. My tooth isn't even discolored. I kind of wanted a second opinion, but he did a cold test on my teeth where he touched a Qtip with cold stuff to that tooth and the teeth next to it, and it hurt pretty bad on the dead tooth, so I guess I believe him. Plus I saw it on the xray. So this Thursday, I'll be having my first root canal.