(Someone on youtube asked me to explain why people become ABDL. This is my reply.)
I've been meaning to write more extensively on this, and I'll let you know when I do, but I'll give you the short version now.
It is difficult if not impossible to say why someone becomes an ABDL. Many ABDLs attribute certain life events to it, but there is no way to tell if things had been different that they would not be an ABDL anyway.
There are many reasons that individuals give for their ABDL origins, and one person's origin story might be the opposite of another.
There are a fair number of people who had problems with bedwetting or even pants wetting. Some of these people were forced to wear diapers because of this, others wanted to wear diapers as a sensible means of protection, but were not allowed to do so.
Some people were envious of the attention a younger sibling received. Others were the "baby" of the family. Some were forced to grow up too quickly, and others were over sheltered. Some came from abusive homes, and some had typical, happy childhoods.
Part of what I would like to research is why do people get fetishes in general. From what I read on Wikipedia, most fetishes develop in early childhood. Assuming this is true you would expect fetishes to develop for objects and circumstances that are normally encountered by young children. For example, young children crawl around on the floor all the time, and feet and shoe fetishes are two of the most common fetishes.