I have a few reasons...
1. I was abused as a child. So the idea of letting someone numb my lower body and then touch/put their fingers/hands into my vagina is NOT HAPPENING. I would rather be in screaming agony than let anyone do that. I am fine with medical examinations etc. SO LONG AS I AM AWARE and able to be in control.
2. I am scared of hospitals. I spent most of my teens in and out of hospital with my terminally ill mother (heart attacks, a stroke, surgeries for heart problem and then later cancer, radio and chemo appointments) and my BP shoots up by about 20 points (diastolic too!) when i walk in the door. No amount of non-understanding people (medical or random) saying "but maternity isn't for ILL people" makes that go away.
3. I want to know if i can do it. With #1 i had a homebirth. It was what a lot of people would call "natural" but i used gas-and-oxygen (entonox 50% mix) for the last 40 or so minutes and had syntometrine for the 3rd stage (for no medical reason - nervous midwife). So to ME it wasn't natural. I didn't ask for either, they were just handed over by the (NHS) midwife who i'd never met before and who kept telling me i wasn't in real labour yet and not to push until DD's head actually crowned. This time i'm super excited to birth again and to go all-natural (luck and nature allowing of course). The "active" stage of my first labour was 89 minutes, so i'm hoping i can handle the intensity, but i have a REALLY great midwife and i'm going to ask her to just leave the entonox at home
I know people say there are no medals, but if i do it that WILL be a medal for me, it means a lot to me to know i can do it (i can't tell you how great it was to have a fast easy birth with DD after years of thinking, because of my abuser, that my body was dirty, ugly and broken). I know after having DD i was super-happy with my experience APART from using the gas (which didn't even help!) so i know that how i'll feel after will be medal enough for me!
4. I have a lot of friends who had natural homebirths, so it's not weird to me, but equally i have friends who have had inductions and elective c-sections too, so whatever happens i have people i can talk to. I was born by elective c-section at 37+4 ("because it was half term and your sister could look after your brother" - my mum). My brother was born at 31weeks by emergency section (with classical cut) after an awful praevia abruption which nearly killed both of them. The other 4 of us were born vaginally, without drugs, even my sister, #4, who was posterior, born at 44+1 after a 96 hour labour. So i knew i could do it, and also that fate could intervene. I was kind of raised with the idea that birth was normally uncomfortable but satisfying and safe, and that if, IF it went wrong in some way, surgery was also a perfectly fine way to bring a baby into the world.
5. Assuming 1-4 weren't the case i can't think of any drug (other than the one i used with DD, entonox) which doesn't carry simply-not-worth-it risks for me and/or the baby. And having gone to all the effort of growing them, i wouldn't risk them over something like pain. Physical pain is nothing to me. It's almost by-the-by. But then i'm very lucky, my threshold for pain is quite high, i've ridden fences on horseback with a broken arm, gone around for 4 days unaware that my wrist was broken, and my birth was not as painful as the 3rd degree ankle sprain i sustained at age 19, luckily (or not, since that's probably why i tripped) i was drunk at the time, which numbed the pain somewhat.