A/N - This is part of a series I'm doing on the first kisses of various ships but I can't add different fandoms to the same story here so I'll publish them separately.
It happend when they took the first case after Sherlock returned from the dead. The case was relatively easy. John didn't even understand why they were on it at all, surly Lestrade can handle this one on his own, but he was still in a bit of a shock from seeing Sherlock again, and he didn't say anything.
Before Sherlock even solved the case, he insisted they would go eat. He ate a lot more since he came back, almost like a normal human being. John kept staring at him throughout dinner. He didn't even notice he was doing this, but after a few seconds of staring, Sherlock would look up at him with a little smile.
"So, what are we doing here?" John asked, still unsure of Sherlock's motives. He trusted him completely but Sherlock has been acting unusually weird ever since he came back to 221B and John wanted to know if he is up to something again.
"Eating," Sherlock answer, looking at John.
"But, you don't do that, you never eat while on a case."
"Well, that's what people do, don't they? They go out, and eat."
From the look on his face, John knew he will not get any further explanation. Sherlock just assumed again they both understood what he meant, so John let it go and went back to his dinner. All he could think about at that moment was how much he missed Sherlock and how grateful he was that he got his miracle.
When they both finished eating, they walked home. They didn't say much, but that was not unusual since their reunion. John wanted to know more about Sherlock's life in those three years, but he waited for Sherlock to bring it up. Unfortunately, he didn't seem to want to talk about it, not yet anyway. Sherlock looked uncomfortable around John, and that was a disturbing look for the world greatest and only consulting detective. John hoped it meant he at least feels bad about what he put everyone through, but he knew Sherlock rarely felt bad about anything.
When they reached 221B Baker St. Sherlock stopped.
"I thought you took the keys?" John said while searching in his pockets.
"Of course I got them," he held the keys out, but didn't open the door.
"So what's wrong?"
"John, I had a lot of time to think while in hiding, and-" Sherlock leaned forward and placed his lips gently on John's.
"Wha- what are you doing?" John said, taking a step back and looking around, checking if anyone saw that.
Sherlock just looked at him, a bit disappointed, and opened the door. John stood at the entrance for a few seconds and then mumbled, "Oh, fuck what people might say". He stormed inside and kissed Sherlock with so much passion, it surprised both of them.