I found this question and it made me think of an appropriate answer and here's my honourest response.
Personally speaking I think it has something to do with psychological reasons, as it brings back memories of the past as most of us can remember someone instructing you to raise your bottom or legs up prior to it being slid under you.
I myself sometimes ask myself to do this when putting a diaper/nappy on whilst laying down,
Speaking of someone else helping you well that brings back memories however since that hasn't happened since I was 9 and a half I can't say how I would feel until it happened because the last time I wore a child's sized diaper/nappy was in 1989.
Although personally I wouldn't mind one bit of someone was prepared to diaper me up like a baby I wouldn't say no as specially if they asked me to lift up my legs and this would be pure heaven although they would have to be my care provider or nurse I do not make necessarily mind which one.
This will probably actually happen again if I go to a AB/DL nursery which plan to do later in my life as I would need to go somewhere I could safely regress without fear of people who don't or can't understand me giving me a lot of negative and highly damaging stigmas.
As for a carer or nanny if you will I would probably ask for one later in life when my medical condition/disabilities gets worse and I need to be in diapers/nappies 24/7 like I was when I was an infant all those years ago in the early to mid 1980’s.
Anyway those are just my thoughts on the subject at hand.