He has a perpetual back itch.
"Mommy will you scratch my back?" Sure hon. Then, as soon as I'm done with a perfectly decent back scratching, he immediately follows with "Daddy! Will you scratch my back?" Which has been known to lead to another "Mommy will you scratch my back?"
It doesn't matter the day, the time, the place, or the provider - he's got that monkey itch. I can't remember a day that has gone by without him asking. I know it won't last forever (unless it is a serious disorder!) so while it does I'll smile and oblige. Kids grow out of these quirks all too soon, and sometimes they become forgotten.