I'm not sure I understand the question. Do you want to know physicall why its painful? Think pushing a bowling ball through a hose.
Thankfully (I guess), most women don't remember the pain of childbirth. As Kieth suggested, because we have difficulty remembering certain feelings from other states than the one we were in when it happened, cause evolution wouldn't be too happy if every woman only had one kid, or none cause they heard how painful it was.