It's just not good Snupin if Sirius isn't put into his place like the rabid, snarling, unworthy cur he is.

Just once, I would love to see Lupin hook up with Snape post-series, and be all guilty and angst riddled (with Snape all smug and triumphant) when Sirius returns, only for Sirius to be all "Eh, whatever. I'm doing Teddy."