Sherlock's best and only friend is sick and no one will tell him what's happening. He reads every veterinarial manual and textbook he can find but nothing he finds in there makes sense. Then something unthinkable happens...
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.