I headcanon that Sasuke might be somewhat of a jumpy person, in his later pre-teen to teen years. Jumpiness and hypervigilance are both common effects of PTSD, so is anxiety, so I can imagine Sasuke getting a little jumpier after the massacre and the more things progress. He flinches when someone moves too fast near him, he starts at the sound of thunder, he jumps whenever a crash or other loud sound resounds near him. It’s an integrated part of his personality that he doesn’t notice since it’s been that way as long as he can remember. He only notices when people point it out in such a mocking manner. Statements such as “Haha, what’s the matter, you scared?” or “Would you fuckin’ relax, I’m not even doing anything!” are among some of the things Sasuke’s heard.
When he’s at home, after several years without any place to call home, Sasuke thinks the habit might be gone. He has no reason to jump, to feel nervous. He’s now an adult (if eighteen is an adult, he doesn’t feel any different from being seventeen) and out of the clutches of his childhood. He’s past it. It isn’t until Naruto happens to slam a cabinet door a little too loud that the old habit rears its ugly head, and Sasuke jumps as the sound jars through him. His eyes close and he prepares for the biting, albeit harmless comment he’s sure Naruto has brimming at his lips. But instead, Sasuke feels a gentle hand resting reassuringly on his shoulder.
“Hey, it’s okay. I’m sorry, I didn’t mean to startle you.”