First of all, luck. And I agree also entirely with subjective, because I loved all three of my labours, they were very different, and they would all sound horrible to some people.
I don't know about genetics, as my mother died before I ever heard about her experiences. But I had big babies, at least two of which weren't positioned well, and with my first I had been completely incapacitated for months with PGP so no exercise. I was overweight with all three. So don't think you have no chance if you haven't managed to be fit and mobile all through.
So, the biggest factor for me was NHS case loading midwives. I had women I knew and trusted with me every time; the same midwife delivered two of my children. They knew me, just as importantly, so we worked brilliantly together by the third time!
Their view on why I enjoyed (more or less) much of labour was that I trusted my body and didn't fear the process. On the contrary, I looked forward to labour and would happily go through it again.
I had pain relief (water and gas and air at various times) but the pain never felt unmanageable. I have birth at home, which meant I had complete control over my environment and no anxiety over or disruption of a transfer.
I was given, by the midwives, the luxury of time. I have been told that if I'd gone to hospital with my first she would have been a C section, as I didn't progress quickly as she wasn't brilliantly positioned. Instead I spent time dangling upside off my sofa, and after 75 hours of full on 'latent' labour, she turned, and active labour was straightforward five hours.
I was always allowed to push naturally, without any direction or inference. It seems I dilate slowly and then suddenly it all happens, so by baby three we all knew the score.
But luck is huge. Plenty of women have what I had and end up with more complicated and painful births. I think the idea of reframing what a 'perfect' birth (if there is such a thing) looks like is vital. If you can come out of it feeling that you were heard, and supported, then I think that makes the biggest difference when you look back.