Sirius silently thanks every deity he's ever heard of for the snide, humourless existence of Severus Snape. He is quite sure that without the Slytherin, he would have lost it months ago. As it is, he can't help but think that the unending sniping and harsh words sneered over the dinner table are the only things keeping him from becoming a gibbering mess. After all, without that, he would be left to the mercy of Remus' pitying glances, or Tonks' happy indifference, or - god forbid - Molly's well-intentioned words.
Because even Snape knows there are lines. Lines not to be crossed, tampered with or even contemplated. Lines to be revered, respected and to not fucking go near. Molly Weasley either doesn't know or doesn't care about said lines, and rushes past them, taunting with the best of intentions. Because Severus Snape is just as protected by these restrictions as Sirius is, and they argue for a reason, even if they would scoff at the very idea of it.
They argue because they have always argued. They have argued since they were in fifth year, when James stuck by Sirius' side and Evans by Snape's. They argue because it seems their only link now to that past, to how they once were. They argue because to not argue would be to concede that they have changed since they were fifteen, and neither for the better.
And the aforementioned 'lines', you ask? It seems an unspoken agreement between them now not to mention the deaths of James or Lily, the years spent in Azkaban or the years spent in service to the Dark Lord. And what does that leave them to argue over? Why, what they have always argued over. And it's keeping both of them from going under, though neither would ever admit it.
AN: short and sweet, yes? Hope you liked it :)