The good Watson had at that time deserted me for a wife, the only selfish action which I can recall in our association. I was alone.
Sherlock Holmes writing from his retirement in Sussex South Downs in The Adventure of the Blanched Soldier, from Arthur Conan Doyle's The Casebook of Sherlock Holmes (1926)
I think usually you don’t hold a grudge against your platonic friends or call them “selfish” for getting married. Usually even if you are concerned about the change of life phase that comes with a marriage, you are more or less socially obligated to congratulate your friend and be happy for them. It would be considered selfish of the grudge-holder to hold the married person’s marriage against them.
To me this line is one of the biggest clues that ACD intended Watson and Holmes to be read as a closeted (and then estranged) gay couple. For Holmes to suggest that Watson’s marriage and desertion of him is selfish implies that they were not just friends, but lovers.
It breaks my heart to think that Watson might have left Holmes because Holmes’ love was unrequited, so I prefer to hypothesize that Watson left Holmes for other reasons, perhaps internal and external cultural pressures that he absorbed throughout his life in Victorian London.