Due to the framed photo of Ms. Irene Adler on his desk, coupled with the fact that he only reverently referred to her as The Woman, Watson was under the impression that Sherlock was in love with her.
Watson was nearly always mistaken. He had a habit of staring right at the facts and interpreting them to fit into his own theories (an unfortunate trait for a doctor).
If Watson were to pay attention, he might observe the following:
1. Sherlock had not thought even twice about leaving Ms. Adler to her own devices after her mix-up with Moriarty outside Baker Street.
2. He had cleaned their apartment more than six times the usual number since Watson and his love of military precision had moved in.
3. He invited Watson (always) to accompany him on cases that he would not ever have shared with another, were the queen herself to order it.
4. His attempts at hygiene were always much stronger on days that he anticipated Watson's company.
5. Besides his cases, Watson was the only thing that could reliably pull him from the opium-induced state he was inclined to sink into every few months.
6. Ever since Watson had announced his engagement, he had employed nearly every method his prodigious mind could conceive of to encourage the good doctor and lady to part ways.
7. After the completion of the insufferable wedding, Sherlock had retreated to Baker Street and immediately pulled all the curtains. The opium was not enough- it was never enough, for it was a poor substitute for love- but he locked himself away with it for an indefinite number of days all the same. And the days were sure to stretch into weeks, if not months. A sequestration amounting to years was not out of the equation.
Frankly, the length of his binge would depend on the rapidity with which Watson returned to pull him out of his depressive stupor.
The good doctor didn't need to feel his pain to know its existence; he merely had to observe.