Severus heard Black's statement and realized two things. First, obviously, he was now flatmates with one of the most wretched people to exist. And, second, no matter where he went, no matter the dimension, reality, plane of existence, or place in time his life unequivocally sucked - he would never escape the mocking torture - perhaps even, his life was only there for the cruel amusement of some high deity, a great, monstrous, cosmic joke.
But resigning to the second fact didn't mean he couldn't fight against the first, his life might be shit, but Severus would be damned if he would acquiesce to Life's current demand he bend over and take it - he would NOT live with Sirius Black.
"Blood hell," Severus spat back, not lowering his wand a hair, "This is my flat, and, as I was here first, clearly, you get your arrogant arse out of my flat!"