John spends three years trying to get over Sherlock. He takes on odd jobs, helping Lestrade with cases and occasionally taking shifts at the hospital. He meets a nice girl, Mary. Time passes. But John doesn’t forget.
Sherlock spends three years biding his time. In spare minutes between toppling Moriarty’s worldwide crime web, he carefully plans his return.
Finally, Sherlock can go no longer without revealing himself
He spots John. The moment is perfect.