Of all things to affect John after the incident, shopping was one he wouldn't have expected. Being in 221B with Mrs. Hudson around, walking though London, the help requests from Lestrade (John didn't know why he kept asking, it's not like he helped the poor detective at all), and the occasional kidnap from Mycroft… Obviously these things would put him off. That was to be expected, seeing as the only reason these people and places were in his life was because Sherlock had put them there. But the first time John found himself in Tesco's, about a week later, he felt worse than he would have ever imagined.
It took him a while to pinpoint exactly what it was about shopping that he dreaded so much. He had never really liked going shopping before, but who did? Eventually he realized that besides the given tediousness of shopping in general, and the chip and pin machines that never got any nicer, John disliked shopping because it brought back memories.
The milk, obviously, was always fun. It was just one of those things he would look at and sigh, the endless arguments over who bought the milk, what kind of milk, and what it was going to contaminate when in the fridge (though in John's opinion the question was what experiment would contaminate the milk, rather than the milk screwing up some experiment).
Once the troubles with the milk had been solved, he would move on to the items on the top shelf. That snarky comment on how John was too short to reach stuff on the higher racks always managed to squirm its way into John's mind, to his annoyance. At least now he had nobody to grumble about when he stretched up onto his tippy-toes in vain.
He would meander on through the store, grimacing at the sugar, sighing at the coffee. Eventually he would make his way to the jam.
Now, let's just get our facts straight first: John loves jam. In his opinion, it's the best thing since sliced bread. He liked every flavor, though he particularly liked strawberry. But now, he couldn't bring himself to take it off the shelf. It was just sentiment, really. The memories were strong.
It had been a peaceful morning, the day he found out about Sherlock's jam particularities. He was making some morning toast and tea, and as he went to spread his jam on his toast, he heard a disapproving voice from behind him: "Grape, John? Really?"
John turned around with his eyebrows raised, only to find the one and only Sherlock boring his eyes into his jam. "What's wrong with grape?"
Sherlock turned up his nose in distaste. "It's no good."
"No good? Do you not like grapes?"
"Oh, grapes themselves are fine. Their texture is simply not satisfactory for jam."
John looked at his flatmate, amused. "So you don't like grape jam then. What kind of jam do you like?"
"Strawberry." And with that, the tall man spun on his heels and stalked off.
The following week, when John went to do the shopping, he bought some strawberry jam home. He though maybe it would be an incentive for Sherlock to actually eat, for a change. The man was so skinny, never eating during cases, and barely eating otherwise. John, being a doctor, was naturally concerned. A man with brainpower like his needed nutrients as to not overwork himself.
However John had expected Sherlock to react to the new flavor of jam, it was most certainly not the absolute, uncontainable joy he had never imagined Sherlock to even be capable of having.
To John's shock and amusement, Sherlock ate nothing but toast with strawberry jam for a week. Also not John's most recommended food for consulting detectives, but at least he was eating. And the smile that graced Sherlock's face every time John presented him with his breakfast was completely worth the health concerns.
Needless to say, strawberry jam quickly replaced milk on the food priority list.