The OP's question seemed to me to be: why is there pain in spite of modern pain relief? Why can't you just opt for pain killers to take it all away? Some answers might be:
labour can take a long time
(depends from case to case)
you may not be able to take a certain pain killer for hours on end, for medical or comfort reasons
(gas made me feel sick after a while)
there are different types of pain during the course of labour- not all pain killers are equally effective for all of them (I found the TENS machine brilliant for the early stages, but useless towards the end)
not all drugs work equally well on all people
there may be medical reasons not to take certain drugs
(e.g. pethidine if there are concerns about baby's health)
unexpected things can happen during labour (e.g. my dd's head got stuck just as the midwife thought I was going to push her out, took an hour to shift her, which obviously altered the pain killer situation somewhat)
different people have VERY different pain thresholds: what seems bearable to you may be unendurable to me
(this is not about being a whimp, or doesn't have to be, it's also about how your nerves report sensations to the brain)
people may have very different expectations:
some people feel let down if they didn't get the labour they wanted, and this affects their perception of the whole thing;
others may feel so relieved that they got through that they forget about the pain very quickly
others still may come to remember the pain as a positive experience (I coped with this challenge)