Both Holmes and Watson see Miss Mary Morstan in a positive light as a woman of refinement and vulnerability. Watson, however, falls in romantic love with her, which colors his view of her. Being in love with her, she becomes an exalted being to him and the center of his thoughts, as is to be expected.
Holmes, however, treats Moss Morstan entirely as a client who has presented him with a puzzling and intriguing case to solve. He resolutely refuses to let his emotions to become involved. As he says to Watson as a statement of general principle:
Detection is, or ought to be, an exact science, and should be treated in the same cold and unemotional manner
After he falls in love with her, Watson experiences anguish as well as joy. He knows if Holmes is successful, Miss Morstan will recover her fortune and be a rich woman. Watson is unhappy about this because he fears Miss Morstan will believe he is courting her for her money, not because he truly and purely loves her for who she is. He is overjoyed at the end when her treasure is lost because it frees him to declare his love.
Watson's love for Mary highlights his differences from Holmes. Watson is an ordinary human, motivated by ordinary, caring human emotions. Holmes, as he tells Watson early in the novel, is, in contrast, motivated by mental challenges. This is why he uses cocaine for stimulation. Holmes states, "I crave for mental exaltation."