Sometimes, John thinks back to his time in Afghanistan and feels a pang of sadness in his chest. London has brought him back to life in more ways than he could possibly imagine; meeting Sherlock, living with Sherlock and falling in love with Sherlock has done more for him than the army ever did, and yet, he sometimes still misses it. He misses looking up at the sky at night and seeing the stars, stretching on and on for as far as he could see against the clearest of desert skies. He doesn't miss the war, the gunshots, the blood, the death or the sand that managed to find its way into everything he owned, but God he misses those stars.
Sometimes, he'll lie in Sherlock's - no, their bed in their flat in Baker Street and he'll look out of the window at the light-polluted London sky and wish that he could see the stars properly. Sherlock never says anything about it, these rare nights where John's heart aches, but he holds John a little tighter and presses soft kisses to the back of his neck.
One day, in the not so distant future, Sherlock will take John away on holiday to a beautiful place where he can see the stars even more clearly than he could in Afghanistan. They'll lie together on a blanket on the beach and Sherlock will impress John with his newly-learned knowledge of the sky and the stars. He'll tell John softly about the constellations, pointing them out one by one, and John will smile to himself and kiss Sherlock's chest. When they return to London, and John looks up at the night sky, he'll remember their holiday; Sherlock sprawled out in bed, barely covered by a thin white sheet; Sherlock sipping ridiculous cocktails by the hotel swimming pool; Sherlock laughing at John trying and failing to get himself onto a li-lo. He'll remember that perfect night on the beach, and his heart will ache a little less.