Well, one thing that can make childbirth a lot more painful than it needs to be is improper positioning of the baby. And one thing that can cause that is staying in a reclining position during pregnancy, especially the third trimester.
Much of the pain of childbirth is from contractions of the uterus. I mean, this muscle doesn't really get a lot of exercise and all of a sudden it is responsible for pushing out a baby.
Also, from my experience, the second child is much less painful than the first, maybe because everything is stretched out more.
But yeah, it's mostly the brain thing. I mean, once the head comes out, the body is a cinch.