Sirius was still irritated whenever someone reminded him of how powerful his captor was. He could live with Voldemort as an enemy. But a “one step below god”-creature was something that his pride revolted against.
“I like hope, you know. But at the moment I would rather fancy a plan. And what do you have against growing old here and die of, I don’t know, a heart attack at 84? It’s better than going back, coming up with a terminal plan and, whoops, have a nice little holiday in Azkaban.”