Draco does it because it's second nature, and he hates that Harry can trivialize that for him. It turns out that Draco hates a lot of things.
Harry doesn't follow any specific pattern for bringing them up, so Draco can't anticipate it. It's a stupid thing to think, when touch is electric and frenetic and desperate, to think that Draco doesn't even like him; he does, he says he does, and maybe he doesn't show that he does, but it should be enough. Draco hates that it's not.
It's easier in the light, when Draco can sneer when he passes Harry in the corridors, and that's what Harry expects. Draco would do the same any other time, but it gets crossed and misinterpreted, so he needs to be on his guard. He hates that he needs to pretend in the dark, where he should be most comfortable, where he should be free. He hates himself for letting that happen.