Draco surrounds himself with a bubble of aloof reserve. His personal space extends for yards around him, so that it's impossible to get close enough to brush him with your fingertips without crowding him. But if you are brave enough to try, if you step in and put a hand on his shoulder, or dare to cup a dangerous cheekbone in the palm of your hand, he turns into the touch immediately. He's unexpectedly kittenish. He craves it, and it's only after touching Draco becomes second nature that Harry realizes he was always like that. When Pansy Parkinson would place a white hand on his arm during Potions, Draco might sneer at her but he never pushed her hand away. Blaise Zambini's fingers on his shoulder as the Slytherins huddled over some piece of mischief at lunchtime never elicited a shudder and a quick withdrawal. Even when Harry himself had punched and clawed at him during their brawls, Draco never flinched away.
Harry thinks sometimes about how starved for true affection Draco must have been as a child, to develop that distancing exterior and still have such a hunger to be touched. He wonders if Narcissa ever hugged him for no reason. He is quite certain that Lucius didn't, and he hopes that Snape found some private moments here and there to offer Draco a bit of comfort, a pat on the back, a gentle healing spell and an embrace in the wake of a Quidditch spill.
And sometimes he thinks about how Draco looked that fateful night in the club in Manchester, when Eliot first dared to twine his fingers together behind Draco's neck and pull him close. Something flickered in Draco's eyes, like the relief of a thirsty man being offered cool water, or the ease of a traveller coming home after a long journey. And then Harry sits down on the sofa where Draco is reading, and pulls Draco's bare feet into his lap. Draco doesn't look up from his book, but he sighs happily and sinks a little lower on the sofa as Harry massages soft circles into the arch of his foot. And Harry feels remarkably lucky, that he's the one who gets to touch Draco as often as he likes and reap the benefits of Draco's sensuality.
Harry wraps himself in a puppyish earnestness, looking at everyone with those great, green eyes, daring them not to love him. And, of course, everyone does. If you weren't paying attention, you might think that Harry's spaniel-esque energy would extend to being touched, that he would metaphorically roll over at everyone's feet and demand to have his belly scratched. But Draco, being Draco, has always paid attention, and he's always been aware that Harry isn't comfortable being touched. When Hermione would lean over and lay a comforting pat on his shoulder during Divination, Harry would stiffen for just a moment until he forced himself to relax into it. His teammates' exuberant congratulations on the Quidditch field left him smiling awkwardly, seeming happy for the attention but uneasy beneath their rough, boyish embraces. Even the uncertain brush of Cho Chang's hand at Hogwarts' ridiculous parties left a residue of slight distress in Harry's eyes.
Draco sometimes wonders what exactly happened in the Dursley home to leave Harry so hungry for attention and approval and yet simultaneously uncomfortable with physical affection. Did the abuse go farther than neglect and exile to a stairwell cupboard? Were there bruises on Harry's slight body in the summers that no one ever saw? He hopes not. He hopes that Harry's reticence with touch is founded only in having gone so many years without it. Perhaps it's just that he never really developed an ability to withstand the intensity, just as people who don't drink milk as children can't digest it as adults. Draco knows that he could ask and Harry would probably tell him the truth. But if Dursley had ever laid a hand in anger to Harry's small, defenceless frame, Draco feels fairly certain he wouldn't be able to control himself. It would be rather ironic to have escaped the Ministry's wrath for all his crimes as a Death Eater, only to be locked in Azkaban for cursing Harry's foster family long after the point where it would have done any good. Harry has never volunteered the information and Draco doesn't ask.
Given Harry's uneasiness with physical affection, it gives Draco more pleasure than anything else in the great, benighted world that Harry's first instinct now is to touch him. He loves how Harry seeks out reasons to caress his bare forearm, and doesn't grow still and cold beneath his fingers anymore when he touches him back. It fills him with a fierce, almost paralysing joy when he rests his chin on Harry's shoulder and without hesitation Harry leans his temple against him in response. When Harry smiles and nuzzles against his palm, perfectly content, Draco can't imagine how anyone anywhere could be unhappy. There's transparent joy where before there was a thin covering of grey. And he sighs happily, and he knows that he is unbelievably lucky to have won this priceless trust and be the recipient of a sensuality so automatic that Harry doesn't even think about it at all.