The first time Yamamoto touches Gokudera, Gokudera nearly breaks his nose.
It's not a conscious decision. The weight of the other's arm falls across his shoulders and all Gokudera's learned reflexes kick in, startle tension along his spine and flood his system with fight-ready adrenaline. All the little differences between them - Yamamoto's greater height, Yamamoto's advantage of weight, Yamamoto's athletic strength - turn into danger in Gokudera's blood, jolt electric instinct through his hands so he's raising a fist to shove himself away to relative safety before he can process the lack of threat in the contact.
It's hard to make sense of, hard to think clearly past the blur of muscle-memory from too many fights and too much necessary self-defense. Gokudera has to backtrack over years, has to reach far into his past before he can draw up recognition of the affection in the physical contact. Even then it doesn't make sense; he can remember the brush of his mother's hand, occasionally his father's fingers ruffling through his hair, but this is something else again. Yamamoto is neither a family member nor even really a friend, hardly more than an acquaintance, and yet he's letting the whole warm weight of his arm hang across Gokudera's shoulders, the angle drawing him so close their hips bump together when Yamamoto takes a step. Gokudera is still trembling with adrenaline, his body tense and heart pounding panic into his blood, but he lets his hand fall, deliberately unclenches his fingers while he tries to shape his thoughts around the possibility that this is intended as friendliness, that this is intended as casual affection instead of the implicit threat that shoved him into alarm.
He can't make himself relax into the contact. It's only a few seconds before he shoves Yamamoto away, snapping something along the lines of "Get off me, idiot" as he retreats to a safe distance. Yamamoto just laughs, unfazed even by this explicit rejection, and Gokudera hunches his shoulders like he's trying to hold the lingering warmth of Yamamoto's touch against his skin.
He doesn't see the way Yamamoto is smiling at him, any more than he notices that he's walking barely closer to the other boy than he was.