Why does Watson break his promise to Holmes in "Hound of the Baskervilles"?Why does Watson break his promise by not going with Sir Henry to...

Watson is put "in a most awkward position" when Sir Henry straightforwardly entreats him not to accompany him to Merripit House.  Sir Henry says,

"Holmes, with all his wisdom, did not foresee some things which have happened since I have been on the moor...I am sure that you are the last man in the world who would wish to be a spoil-sport.  I must go out alone".

As it is apparent that Sir Henry has some very private business to which to attend, Watson is at a loss at what to do.  While he is debating as to the best course to take, Sir Henry picks up his cane and is gone.  Watson agonizes over the situation a little longer, and then finds that his "conscience reproache(s) (him) bitterly for having on any pretext allowed (his charge) to go out of (his) sight".  He then decides, belatedly, to overtake him, and "set(s) off at once in the direction of Merripit House". 

Although Watson hurries as quickly as he can along the road, he does not at first see Sir Henry, and, fearing that he has "come in the wrong direction after all", he mounts a hill from which he can have a better view of the path.  He spies Sir Henry about a quarter of a mile ahead, walking with a lady who can only be Miss Stapleton.  Feeling guilty for spying on what is obviously meant to be a romantic liason, Watson watches as the two talk for awhile, then are interrupted by the approach of Miss Stapleton's brother.  An argument ensues, and when it is over, Watson's sees Sir Henry walk away alone and dejected (Chapter 9).