Why does Watson say that Holmes' instructions not to let Sir Henry go out alone may be hard to do?

Watson believes it would be difficult to be the third wheel in Sir Henry's blossoming relationship with Miss Stapleton.

From the first moment that he saw her he appeared to be strongly attracted by her, and I am much mistaken if the feeling was not mutual. He referred to her again and again on our walk home, and since then hardly a day has passed that we have not seen something of the brother and sister. They dine here tonight, and there is some talk of our going to them next week. One would imagine that such a match would be very welcome to Stapleton, and yet I have more than once caught a look of the strongest disapprobation in his face when Sir Henry has been paying some attention to his sister. He is much attached to her, no doubt, and would lead a lonely life without her, but it would seem the height of selfishness if he were to stand in the way of her making so brilliant a marriage. Yet I am certain that he does not wish their intimacy to ripen into love, and I have several times observed that he has taken pains to prevent them from being tête-à-tête. By the way, your instructions to me never to allow Sir Henry to go out alone will become very much more onerous if a love affair were to be added to our other difficulties. My popularity would soon suffer if I were to carry out your orders to the letter.