I think I might have discovered where ABDL comes from. Might.
First and foremost, by far the most vulnerable time in anyone's life is during early childhood (birth to about 6yrs). It is when the child's psyche is formed; thus, it is susceptible to being twisted and broken far too easily. Pardon me if I'm overgeneralizing, but I think this is where our 'kink' comes from.
Allow me to explain this. After having a look at a few ABDL's brief childhood stories (which were all on this site), I've concluded that most ABDLs are born out of emotional, mental, or physical trauma as a way to cope with them.
I've noticed that a good number of ABs on ADISC have said that they suffered from parental or otherwise emotional abuse at some point in their lives. So, they had to find a way to deal with it. ABs, though, dealt with it by wanting to become a carefree baby again who didn't have to deal with this abuse as a way to combat it. This makes logical sense because it was born from a trauma and, because it was placed in their developing psyches, has stuck with them for all their lives.
As for physical trauma, I'm talking about mostly injuries and sexual abuse. The latter is also explained by the above paragraph as a way to cope, so I'll get to the first one. Physical injuries, particularly ones that would make a young child bedridden for a while, might require a possibly already potty-trained child to wear diapers again due to being bedridden. The child most likely began to view them as a comfort (if they took that train of thought) and thus led to ABDL. Additionally, but somewhat off-topic, IC people who are also ABDL learned to like their diapers and began to view them as a comfort in a similar fashion.
Lastly, mental trauma. This is the road I think led me to the ABDL world. Since you also develop fears of things that scar you in your early childhood, I think ABDL sometimes grows out of them in a similar fashion. Take me, for example. When I was in the vulnerable age group, I had the dumbest nightmare ever, but it scarred me for life. It was about a toilet that ate me whole. Then, I became afraid of toilets that had any kind of autonomy, specifically autoflush ones (a delicious source of Jump Scares while you're peeing!). They really were the scariest things EVER when I was little! In other words, I was mentally traumatized by a nightmare because, again, of a delicate psyche. I probably wanted to go back to diapers so I wouldn't have to deal with them, and thus my love for diapers was born.
All in all, I think most ABDLs became what they are as a way to escape these traumas and cope with stress. Now that I think about it, maybe ABDL is nowhere near as depraved as people who are afraid of us make us out to be. So, maybe we're not so abnormal after all!
But hey, that's just a theory. A GAME THEO--no, I'm not gonna steal a page out of MatPat's book here.
So, what do you guys think?