Long winded post incoming. TL/DR: Bladder difficulties + strict Christian environment + bad explanation of sex + DailyDiapers
I grew up in a very locked-down Christian community. The adults displayed no sexual/physical intimacy in front of us and we had no real contact with or understanding of the outside world. We knew nothing about sex whatsoever other than that asking about it would lead to bad things.
I also had a very oversensitive bladder as a child that kept me in a near permanent state of having to urinate. That, plus an intense phobia of public restrooms, led to several traumatic experiences wetting myself in front of people throughout elementary school. Also, there was increasing disdain from adults towards me for constantly asking to use the bathroom. So I think these things psychologically predisposed me toward an interest in diapers as a release from that physical and emotional torment. But there were several incidents during this time where I came across diapers in normal (and even two early sexual) contexts and I didn't have the slightest interest in them. So it wasn't something I conceived of at the time, but at least the groundwork was there.
Then I was hit hard by puberty at the end of the 3rd grade. Was already a shrimpy, bullied, nervous wreck of a kid. Severe acne, body odor, voice cracking, hair growth and parents who not only made no attempt to help me, but actually viewed my emotional preoccupation with my appearance as an evil, made me a complete outcast. Queue the self-reinforcing emotional spiral from there.
Along with that came intense sexual arousal which I had no basis for comprehending. Even though it was mentally all-consuming, it somehow didn't translate into anything physical for years (e.g. no erections or stimulation from anything physical). At about this time, the school was mandated by law (I think) to give us a sex-ed class. but being hard-line Christians, they went about it in the most obtuse manner they could. Early on they described wet-dreams as "starting to have bed-wetting accidents again" or something like that, and then the class of teenage boys descended into chaos. The only thing I took out of it was that sex involved some kind of involuntary release of fluid from the penis akin to involuntary urination. So I started spending hours each night at the toilet trying to induce myself to have an accident. It was excruciating, but in the absence of anything else it was the only possible glimmer in the dark. When that didn't work, I figured it must be because there was nothing really accidental about using a toilet. So then I tried in the tub, then with my clothes on, then in bed, then at my desk, then in physically confining conditions, and so on. Even tried messing a few times. Thankfully I never tried to do it in public and was never caught (as far as I know at least. I've recently become convinced that I have dissociative amnesia that has blocked out a substantial amount of my memories of this time).
I also got my hands on some Victoria's Secret magazines at this time, which was my first exposure to the female body or anything sexual. So I'd just spend my nights staring at those catalogs while trying to induce an accident. I could feel that I desperately wanted sexually developed women (who would therefore be older sisters, mothers, teachers, etc.) to physically interact with my nether regions in some way, but I didn't know how or why, nor how urinating could possible fit into it. The only time women interacted that closely with anyone, as far as I knew, was with their very young children.
Then one day my dad made a joke about adult diapers, which I did not know existed up to that point and had somehow never considered. I raced off to the computer, immediately stumbled across the DailyDiapers website, and my fate was sealed (I seem to recall that the very first thing I saw was a poem about a girl orgasming - a word I didn't even know at the time but got the gist of - while losing control of herself in a diaper Infront of her classmates). Everything fell into (an unfortunate) place: diapers formed the connection between urinating and physical contact with sexually mature women. Women keep you in diapers because your urge to urinate/cum (which in my mind at the time were the same thing) are uncontrollable. They find your inability to control yourself, especially in their presence, irresistibly adorable and shower you with physical and emotional affection. It wasn't until years later (probably around 15 years old), and a chance encounter with a vibrating appliance, that I had my first orgasm and came to understand what sex actually is. But by then my brain had been thoroughly mis-wired and there was no going back.
So that is my canonical explanation. It fits with all the memories I have and the dates I can firmly pin down. But there is one hitch: I had a (verified) preoccupation with keeping some of my favorite toys in diapers that didn't fizzle out until approximately age 8. And I seem to recall having strange feelings of pre-sexual physical/emotional intimacy about it. That would make sense given what I said about the oversensitive bladder, that I was using the toys to act out what I (not fully-consciously) wanted for myself. So maybe I was cursed to develop this fetish from the beginning anyway.