Sten really appreciates it if you understand when he's joking in Origins, actually. To paraphrase:
Warden: What were you doing in that cage?
Sten: Sitting, as you saw. I thought that was obvious.
Warden: Very funny.
Sten: Thank you. *slight smile*
So Qunari definitely have a sense of humor in there, somewhere. It's just rooted in sarcasm, and they don't actually change their facial expression much so it's difficult to tell (if you're not into that sort of thing).
One theory is that (from TvTropes? Can't remember) is that this is a race that values absolute mastery. Since many of them don't speak Fereldan very well, they're embarrassed and try not to let their lack of mastery show, hence the one-word answers. Consider the difference in how Sten speaks, versus the rest of his unit in the Fade. You're probably hearing what he heard, not what was actually said, during that scene.
And every other thing Sten says is concentrated troll. My favorites being "I posted riddles to passing travelers, and offered them treasure in return for correct answers" and "I picked an object, and tried to think of how many other objects I could name started with the same letter."
The second was probably him trying to perfect his Fereldan, come to think of it.