Why was Sirius leaning so close? If Remus didn't know any better, he'd think Sirius was trying to flirt with him. Good thing he knew better; he might have fallen for it.
And yet still, even knowing that Sirius hadn't meant anything by it, Remus found it very difficult to take a step backwards, despite the hurt and the distrust. He didn't know what Sirius was playing at and although the knowledge that he was definitely playing at something did make him uneasy, he wanted it to be true. Remus had never wanted anything more than what he was getting right now, but that didn't mean he didn't know another joke when he saw one, no matter how unfunny.
Remus took another step back, holding the door open and waved Sirius in without another word; he had some explaining to do and Remus wasn't feeling the least bit inclined to help him out with it.