Greg wasn't sure what to answer to Sherlocks first question. He didn't know himself, to tell the truth, why he was helping him lke this. He hadn't thought about ti for a second, for him it felt like the thing to do. Especially without John around to keep an eye on the detective. And with his injuries someone had to make sure and help him with whatever he was stubbornly doing, so that they wouldn't get worse. He didn't say any of that though, instead, he just shrugged his shoulders. "Why should I not?" he instead asked in return.
He watched, a little worried as Sherlock held his head. He wasn't sure whether the doctors were still watching over him for any signs of a concussion. But once awake, there was nothing he could do about Sherlock leaving the hospital, one way or another. And he would like that to be with his help rather than without.
He laughed when Sherlock mentioned that time, when he'd had the flu. He could laugh about it now, at least. Back then he had just been so annoyed, he'd almost been furious with the other man. He really didn't know how John ever put up with that attitude of his. "I remember." he answered him, shaking his head to himself. He really hoped this would be different. "You will have to though. Or your injuries will take longer to heal and you won't be able to do you work properly until then." he hoped, that this thread was enough to let Sherlock see reason. 
"Okay, so let's get you out of here." Greg said and wandered outside, snatching he first wheelchair he could find and bringing it back into Sherlocks room. Before the other man could protest, he said sternly: "That's the only way out and you are very well aware of it."