Sherlock missed John immensely. He didn't know how much longer he could continue playing dead. The love of his life was out of his life and he could do nothing about it, not for a while at least. Not until things settled down could he return to Baker Street to be with John again.
He remembered better times, before the mess with Moriarty. It was a more boring life, sure, but better nonetheless. Sherlock would sacrifice a life of excitement in the blink of an eye in order to be with his beloved.
He thought about that time in the café, just before their first kiss. John had bean wearing the maroon sweater that was frayed in a few spots as a result of the dangerous cases they worked on together. His smile had been endearing, with a small blue stain from his blueberry muffin. His limp had been less pronounced that day, as if just by being around Sherlock made him notice the pain less. Sherlock remembered all the details because they were special to him. In his powerful brain, he only stores things that are important, and he made sure to remember every single moment of time spent with John.
And then there was that kiss. It was drizzling lightly outside, and John blushed when he realized that he left his compactable umbrella inside the café. Before he could turn all the way around to head back inside, Sherlock grabbed his hand and swiveled him back facing him, pulling him in close for that perfect kiss. The rain continued falling, but they didn't care. They had both been waiting so long for this moment, not even realizing it was possible, until it was, and suddenly it seemed silly that their flat had two separate bedrooms.
Sherlock knew he had to do something, to find John and show him that he was alive. John would be so relieved. Furious for disappearing for so long, but then relieved again. It was dangerous to go back, even after Moriarty died, but it would be absolutely worth it. He no longer felt complete when John wasn't around.