It started slowly. Sherlock came back, John forgave him - of course he did, and they fell back into rhythm, as if nothing had changed.
Gradually though, it got increasingly difficult for John to watch Sherlock fly brazenly into the air. The first time, he had leapt from one roof onto a lower one below. Arms out, mop of hair fluttering in the wind. John froze, heart thudding painfully in his chest. He caught his breath and leaned over the wall, eyeing Sherlock grinning and waving cockily on the flat roof below.
From there, it got progressively worse. One day John came home to find Sherlock leaning over the railing at the top of the stairs and couldn't halt the mental picture of his best friend, lying shattered and bloody, on the landing. Trembling, he slumped against the wall.
Sherlock, for all his powers of observation, overlooked John's increasing panic until one day it just got to be too much and John exploded, finding Sherlock dangling from some scaffolding at a crime scene.
"Jesus, Sherlock. Just stop it. You are not a bloody monkey, keep your fucking feet on the ground." John gasped, hiding his face in shaking hands.
Of course, John thought bitterly. Leave it to one fractured, co-dependent ex-Army doctor to go and get some displaced form of bathophobia.
Bathophobia is a fear of deep, empty voids - staircases, caves, the gaps between tall buildings. I'll leave it to you to figure out why this bothers John so much now. /angst