"This will lead us nowhere..." A very cheesy moment, a very cheesy line for Snape to tell. (from "Pure Blood", my fanfic written in Greek and waiting for some Greek-speaking volunteer to translate it in English).
Of course there's no such line in my story (there are much much worse) but it suited the scene, it could be something he was thinking that moment (hence the gloominess) but never dared utter it.
In the story Snape was always the grumpy, reluctant one and Lucius the joie-de-vivre type, spending money on silly things, enjoying life and living for the moment (the opposite of Snape - that's why they make such a nice couple). I think most shippers of this pairing have the same ideas for their characters and how they would behave in the relationship.
Severus Snape and Lucius Malfoy (c) J.K. Rowling