It is often the case that what one feared most in youth becomes a fetish later in life.
Having grown up in the era of penicillin shots, my greatest fear when going to the doctor throughout by childhood and adolescence was having to pull everything down to my ankles and bend over the receptionist's lap to get one. Even when going for a routine medical examination I got booster shots the same way. It wasn't the shots I feared so much as how I had to get them because I deemed injections in the bottom childish, especially with the receptionist holding me across her lap and presumably watching. The older I got, the more humiliated I felt but, curiously, I often fantasised about the experience when no shots were imminent.
House calls normally ended with me rolling over in bed and having my bottom bared for an injection. Although I was spared the embarrassment of bending over a receptionist's lap, we had an au pair girl who always seemed to be just outside the bedroom, capable of hearing what was happening and perhaps even seeing through the open door. Once my adolescent girlfriend was visiting when the doctor arrived. As the examination was above the waist only, he allowed her to stay in the room and I felt relieved because I feared asking her to leave could mean a dread shot where I sit and she might perceive the reason. Even worse, he told me to roll over with her still there and she watched as my bottom was bared and painfully injected. After he left, however, she excitingly consoled me with tales of her own injections back there. Recognising a mutual fetish, we soon began playing doctor and every session involved buttock injections realistically simulated with sterile pins. Although enjoying the thrill of getting as well as giving those play shots, we both remained frightened of the real thing - regarding buttock injections as childish and getting them in the arm as a rite of passage. When there was a problem with the oral vaccine and we had to get polio shots, we were both afraid of where we would get them. She was a year older than I and got hers first, in the arm, but then terrified me with the news that her cousin of exacly my age got his in the bottom. I feared that the rite of passage for shots in the arm might be between our ages and that getting mine back there would make me childish by comparision with her. I decided that I had nothing to lose by asking for mine in the arm and, to my surprise, the doctor agreed with the caveat that my "usual" (presumably penicillin) injections still had to go in the bottom. He gave me the second and third polio shots in the arm over the course of a year, after which there was a two year gap before the final dose. A couple of months before then I had to get a tetanus booster from his nurse and was afraid of where I would get it because I had until then got booster as well as penicillin shots in the bottom. I discarded the idea of asking for in in the arm because her cubicle was merely screened-off from the waiting room and I was afraid of the embarrassment if waiting patients heard her refuse. My only hope was that she might ask me where I wanted it, but she didn't. I was embarrassed lest those outside hear me told to pull down my pants and bend over so, fearing the worst, I did so before being told and "voluntarily" took the shot in my bottom. When I went for the final polio shot from that beautiful nurse, I looked forward to her treating me like an adult by injecting my arm but, to my horror, she told me to pull down my pants and bend over with everybody listening; refusing to believe that I had the previous three in the arm because of my booster shot behaviour. I reluctantly dropped trousers - too reluctantly for her patience because she pulled my pants all the way down herself. Humiliating though it was, I found myself mature enough to appreciate the sensuality of a pretty girl baring, swabbing and even injecting my erogenous posterior. The experience brought my fantasies to reality and I have ever since enjoyed injections in my bottom, especially when given by attractive nurses.
My fetish is now such that I feel nostalgic about shots across a receptionist's lap and I genuinely wish that I could relive the anxious thrill of ogling her shapely mini-skirted legs while she watched me injected in the bare bottom.