TLDR: Received unliked enemas then enjoyed other forms of anal stimulation then enjoyed enemas.
I HATED the first enema or two I received as a very young child, even more than I hated receiving rectal thermometers. I remember vividly, though I could not have been much more than two years old, dreading doctor visits because they'd always take my temperature in the "baby way". HATED it.
But then I remember the first enema I didn't hate, when I was probably 3 or so. It was probably the last enema I received as a child from someone else. Thats not to say I enjoyed it - more like I dreaded it when Mom told me I needed one, but unlike prior enemas when I'd been forced, held down, etc., this time she calmed me down, talked me into cooperating, and soothed me as it happened. And I had to admit I felt better afterwards. A rectal temp received from Mom was less traumatic as well. So the last enema and rectal temp were more a matter of being less traumatic than previous experiences than they were a matter of budding erotic interest - at least consciously.
But a seed must have been planted. It was only a few years later that I started, as the say, "exploring my body". At some point - pretty early on - this involved giving rectal thermometers, enema nozzles and douche nozzles (which I thought at the time were just adult enema nozzles) another go. Shortly thereafter I was routinely giving myself enemas in the rare instances when I had the house to myself for a while.
I was also learning to masturbate, but initially that was a separate thing. However, not too long after I first experienced orgasm, I quickly added anal stimulation to that process. The blunt end of ballpoint pens felt good and were inconspicuous on my nightstand, so that was my go to, but when I had the privacy, thermometers and nozzles got their turn.
Pretty soon I added actual enemas when possible, and that combination sent me over the moon. I lost interest in rectal thermometers, however. I wouldn't say no if offered, but I never find myself thinking about them or incorporating them in sex play.