I was just discussing with my Mommy last night one of my first real memories being somewhere in '99-'00 when I was 5 or so in daycare, and seeing a pack of diapers and just being inexplicably drawn to them. I had zero idea why, but in my 5-year-old head, it was really simple. Maybe it was because I was a bedwetter and thought I wouldn't have to worry if someone just put me back in diapers. I was attending what turned out to be a really abusive daycare situation, so maybe I sought safety in the diapers from one of the staff who used to bully me. Either way, I was five the first time I snuck away, out of adult sightlines, grabbed a diaper, and tried putting it on.
I didn't entertain it through childhood really until my brother was in pull-ups. I tried a few on, peed in a few, they were alright, but I wanted diapers, and the CVS store-brand diapers kept leaking -- and so my first case of Bambinos Classicos was on its way sooner than later. This had to be, I dunno, 2009? 2010? (age 15-16?) I wore those to high school sometimes, but a combination of judgmental peers, being rather active, and hard high school desk seats stopped that from being a full-time thing. Once I could drive and was leaving high school, I found a medical supply store south of the city and was introduced to Tena and Molicares (cloth). I spent my whole first year of community college in diapers more often than not. Not quite 24/7 but wore to all my classes and meetups. I was just another college kid in comfy sweats -- and, unbeknownst to literally everyone else, a thick diaper underneath.
In my adult life, the ABDL industry has exploded and I have also found a relationship with someone who takes great joy in my ABDL/pup side -- all of it -- including the dirty diapers, changing, and just everything in between. I've been diapered and collared 24/7 since at least the beginning of the year, but I kinda forget. My life just kinda began making the transition to the way I always wanted to live once I was out of my house and with someone who embraced me.
TL;DR - I think (and Mommy thinks) it's a combo of subpar parents who didn't give me much attention and an abusive daycare experience being one of my earliest memories where I sought out diapers as a reliever, for some reason. When I was older and my brother was pull-up age, I think it was me trying to regress out of spite because I was left to raise my brother. When I was in college, it was a very sensory, self-soothing thing to be in class, diapered, with big comfy sweats on. It was the only way I could pay attention fully in class. As an adult with OAB and IBD, and a very very supportive spouse who prefers me diapered, I quite literally don't feel comfortable not diapered, most of the time, and try to keep "air out/underwear time" to an absolute minimum.
Addendum: I don't think it's something that is simply connected to the childhoods we all have. There are too many variables. Too many people with idyllic childhoods end up ABDLs for it to be as simple as "Abusive/subpar childhood with less attention = ABDL" or "Abuse = ABDL". I think, if I had not entertained the goodnites, I probably wouldn't have bought CVS diapers. If I hadn't bought CVS diapers, I wouldn't have gotten an appetite for ABDL diapers. If I never experienced the beautiful sensory satisfaction, safety, and security of premium ABDL diapers, I would have never been able to envision being in them real-deal full-time for the rest of my life. I think it's a bit of a rabbit hole and you choose what avenues to develop down. Yes, the first seed is planted in childhood, but I don't think it's necessarily one seed in one place, and I don't think the seeds are only planted in your earliest years.