I have heard that the word "pain" is counterproductive during the labour. I cannot agree more. Pain means that someone is getting hurt. Pain can be a signal from your body that something is wrong. Pain is not what you should feel when your baby is being born.
Natural birth is often compared to climbing a steep mountain. It requires much effort, but it is beautiful. You start to feel tired at some point, but you go on. Your muscles might start to ache, but you do not feel it - you are on your way. Your body starts to secrete endorphins - hormones of euphoria and happiness. At some point you might get more tired than excited, but it is too late to turn away, you need to go. You might get a couple of bruises, you might trip now and then. And you get there, you sit down, look around and rest. Fulfilled, satisfied, happy. A part of you realises that your body is tired and aching, but it seems very irrelevant at the moment. You made it.
Now imagine that you hate mountains. You were taken here by your friends, but you don't like walking, let alone hiking. You are not really in shape, you got short of breath already after the first mile. You pant, your body screams, you are in agony and you know that it is still far away. Your friends literally drag you, you painfully feel every stone your shoes. You make some twenty yards crawling on hard rocks that leave you bruises. You don't care whether you get on the top or not, you don't care about anything, you just want the suffering to cease.
Climbing the same mountain can be pain-free or painful. It will depend on how you feel on a given day, on whom you meet on your way. However, it mainly depends on you - on how prepared you are. And it all starts in you mind.