I don't know. This is kind of a complex question. I starting liking diapers at an early age (4? 5?) and can't think of anything either of my parents did to kickstart that at that time.
I think it was a couple of things...
Most kids look forward to growing up, but I was always scared about it. I saw growing up as being able to acquire things I wanted currently but no longer desiring those things. I remember being extremely frustrated at the idea that I, for example, could not afford toys that I wanted, and relied on adults to buy them for me. I knew one day the toys I enjoyed would be a drop in the bucket monetarily, but once they were, I'd no longer want them. I'd want more expensive things that I, at the time, found immensely boring. One day... I would buy a couple hundred dollar vacuum cleaner... and like it...
Teens also freaked me out. When I was little, it seemed like as you got older your intelligence level increased... until you became a teen at which point you became retarded for a few years before starting an upward climb again. So I was terrified of becoming a teen and that happening to me. Having watched it happen to so many people (and general observations on "typical teens") I thought hormones just made you crazy for a short (hopefully) period in your life.
I was also treated in many ways as an adult when I was very young, and many people said that I acted much older than I was. (When I was 8, a college friend of my mother's told her she felt intimidated around me, and that she was afraid she say something around me that would make me feel she was stupid. Another friend said I reminded her of her father.) I was around adults more than children and often preferred them to people my own age. Most of the time, when young kids speak to adults, adults will acknowledge that they're speaking but are really ignoring them (patronizing). Not that this never happened to me (I would notice it and simply stop talking if someone did this to me), but most of the time I'd partake in adult conversations as a sort of honorary peer. So, I think I clung to youth discretely through infantilism as a way to counter aspects of myself that were the opposite of infantilism.
It's actually mildly humorous when you think about it. I was into being a toddler pretty much... while I was a toddler or immediately afterwards. The big feature that stood out about me as I was growing up was that I was a "little old man". Now that I'm an adult (almost 30), the "little old man" features don't really stick out so much anymore. Now it's my fondness for stuffed animals, blankies, pacis, etc. I'm basically a larger, more experienced version of myself when I was 5 and the things that were odd about me are becoming normal whereas the things that were normal about me are now odd.
Ironic (don'tcha think)