Why is it so painful to give birth?

Two answers. The first-physiology, it is not that easy to get a cantaloupe sized baby head to pass through your vagina. Just think how painful a large stool can be and quintuplify it. The second- part of the divine plan and miracle of birth-the answers to which we are not privy to. I speculate that this magnifies the maternal child bonding magic.
Many reasons. Gravid (pregnant) uterus causes lower back pain. Your fetus's head pressing on your cervical and vaginal wall can cause pelvic pain, vaginal pain. Also your fetus's body dilating your cervix and vagina results in severe pain.