Sherlock loves piggyback rides. The detective’s fondness results from spending the summer of his fifth year being carried around on Mycroft’s back, quite literally over the river and through the woods.
John is a lot stronger than he looks, a fact he feels no pressing need to prove, yet a truth he does wish were a little more widely known. All this is by way of saying that if Sherlock ever wanted a piggyback ride or two some summer day, John Watson would be more than happy to oblige.