I guess suppositories were the very first thing that got me interested.. A bit difficult to tell exact as quite many things probably contributed, but suppositories are definitely one. I probably got them since I was a baby, but I remember first time having some special feelings about them when I was 6 or 7 years old.
I only got them in one very specific situation: when I was really sick and had high fever along with headache and risk of vomiting. Does not sound that regular, but as a kid I typically got a bad flu or something else like that a few times a year and when I had high fever I automatically had bad headache and started vomiting. So it was time for some suppositories a few times a year all the way until I was 10 or so.
I still definitely remeber how it all happened, so it must have been a strong experience already then. It was probably the fact that it was always my mother who gave it to me and the way she put it into me in the "diaper position" (which still today really turns me on!). She always did it in the privacy of my room, but she insisted me taking down my panties completely (I typically had only pyjamas because I was sick already) and told me to lie on my back and "pull my legs to my chest" (I still remember those exact words she used).
It was funny because on the occasion it always felt embarrasing and when I was older I even cried once when I had to let her do it, but afterwards it felt exciting and I was thinking about it often. I was very aware of my genitals already when I was 6 or 7 (I had started masturbating around that age) and even more so when I was 9 or 10, and I remember thinking how odd but also exciting it felt to have my mother see my genitals and I felt really exposed in tha position already then. So I guess it must have been quite exciting for me as a small boy as I still recall all this clearly..