Teenlock. The reason why Sherlock Holmes cuts himself off from emotions and anyone else, why this man is so cold and rude to anyone he meets, is to protect himself from the hard truth: That in the end, everybody leaves.
Sherlock is always so concerned with keeping up the impression that he is beyond mere mortals. Cold. Distant. Never affected. Not in the least interested in pursuits such as emotions or sexual relationships.
John would like to call his bluff. So he gives Sherlock a taste of his own medicine: by running an experiment of his own.