Fear can stop the labour, or at least can block the euphoria hormones and cause pain. Fear is not a natural part of birth. When an animal is in labour and something scary happens, the labour will stop and the animal will run away, looking for a better hide and for a better moment. Women are not very different in that matter.
Your body needs to know that this is a good moment to open and let the vulnerable baby out. When you are anxious, your body does not believe it is safe and resists - finally the baby will be taken away by force and this must hurt.
We fear of what we do not know. Read as much as you can, but stay away from scary stories of your cousins. Read about waterbirth, homebiths, pain-free births, unassisted births. Learn how clever our bodies are, how our instincts work when given a chance.