John Watson was the only sincere smiling face Sherlock had seen in years. He put it on his wall in yellow paint so he could always feel his presence. But knowing that the smile could never love him the way he loved John pushed him over the edge, and he shot it again and again to try and shake the sentiment out of him. When John found him like this, he shouted, "Bored" to distract him from the tears filling his eyes.
One Word: Johnlock.