I'm pondering that a lot of the time. What is it, exactly, that starts it? What is it truly about? What made me this way? For most people wearing an oxygen mask or being put to sleep isn't really a pleasant experience, or at least not one that somebody would like to experience, let alone be sexually aroused by it.
In my experience, my parents watched ER quite a bit when I was little. I did have some fascination with oxygen masks and generally oxygen delivery as a small kid, but it's after I met a girl with an extremely rare, genetic neurodegenerative condition at age 8 that my interest truly got upgraded to a secret obsession. For the first time in my history as a child, I knew, I had before my eyes, something that no child should really experience if they want to stay innocent: knowing that someone your age, as young as 8, can be very sick and die. It's also at this point in my life that I started being obsessed about death and dying, and probably around there that I truly started to become a hypochondriac (because yes, I have some serious health anxiety tendencies, which in a way turned out positive because I know a lot of things about diseases and medical stuff). Probably it traumatized me in some way.
The second and last time I saw her, she was wearing an oxygen mask instead of the nasal cannula she had on the first time. I wondered how it felt like to breathe with one. I was hell into it. I would research them privately, terrified that my parents would find me out. The taboo was there already. It was charged more than another interest. I fantasized about going to the hospital, which I was afraid of because I was afraid my parents would take pictures of me and bring those memories up later, and having the mask put on my face. "Hi'iaka, you're going to have this mask over your face, it's to help you breathe, honey". For many years I've been wanting to acquire a few of them, not just for play, but also... just to hold the thing and study it, in a way. Try to understand what truly attracts me to it. I'd do just like my character Namaka however: I'd keep all of these supplies in an inconspicuous box/chest, or better, something I can lock or otherwise have inaccessible to others. It's something that is mine and only mine, and that is too viscerally close to my core to have anyone in my life truly know about it. It won't happen until I have my own place though. Which could be... in a long time. If ever.
It should be noted that my home environment as a kid was pretty restrictive despite being apparently let loose (few rules, but the limits were non-negotiable and hitting them meant an emotionally violent outcome that was enough to scare me into not going there). Suffocating, in a way, even I was told at a very young age not to trust anyone, learned a lot of the time to shut my emotions or needs down... to hide my interests, my life, to only show a fraction of who I truly am, because the real person wasn't wanted, only the "child" was...
...and what exactly is the purpose of an oxygen mask? Helping a patient breathe, in a medical setting. Give them extra oxygen because either they're not getting enough from the 21% in room air, because they need help waking up from surgery, or because their body is otherwise in need of more for a while.
Maybe for me, it's about having something help me breathe. Survive. Stay alive when I cannot support myself. Literally putting myself on life support. I was a child that had to deal with her emotions, needs, and with her own personality all alone. Lift myself up at an age where you are weak and dependent. And like some have mentioned here, this emotional thing got intertwined with sexual pleasure as I grew older. It's often in times of stress that my fetish seems to flare up dramatically, I note. I also fantasize about giving myself some treatments to relax at home, even though oxygen toxicity would probably be a risk and I could end up dead or with half a mind (who knows, though, if I can't find a way to make it as safe as possible). Just imagine finding me dead half-naked, clearly out of oxygen toxicity rather than carbon dioxide like one would expect to find a dead person in this context, and outing me as a well-compartmentalized medical fetishist who ended up breathing herself to death. Talk about outing myself in the worst way possible. At least I wouldn't be around to be embarrassed, but... the idea is still terrifying, albeit hilarious in the same breath (pun entirely intended).
For anesthesia, I believe it's the curiosity of what it's like to disappear out of consciousness for a while. It came later than the regular oxygen masks, a year or so before puberty truly hit me. The reason why they give you oxygen is because you stop breathing (flirting with death?) once the anesthetics kick in for real and your body needs some reserves to last until it's intubated. But it's less about falling asleep than seeing that mask slightly fog up, especially if your lips are a bit parted, just letting some air pass. That thing looks closed and without the air coming in you'd be suffocating within a minute, but there's oxygen coming in. It's a small space, a tiny space, but it's full of what gives you life. What if in a sense, it's a little refuge? A "breath of fresh air", or a "bulle d'oxygène" (oxygen bubble, literally) like we say here to say a release, an outlet?
My therapist told me I grew up in a cold environment. I told her that my source of warmth was a lighter (metaphorically, of course), and she commented on how that warmth was artificial. It might also come down to that... artificially sustaining my life, because I feel there are moments I might just die on the spot from how overwhelmed I feel having to carry my own (metaphorical) weight (because ironically, I'm on the petite side).