Do Death Knights ever...

[Stormcall]

We all secretly love ponies.

No, I can say with utmost certainty that we do not.

Mine smiles after I'm the one left standing after having 2 or 3 guys trying to kill me at once.

[Requiemcliff]

Meh, my DK was a bit more serious than the character before him, but he wasn't dour or anything.

(My other DK was delightfully insane...I really have to play him more...)

[WeaponXAnimosity]

DK's may be undead but they're people too. They can smile but in the roleplay aspect of WoW, a lot of people like to default on the "has a stick shoved up their cheeks" persona because it seems more epic and well, it's easier. Than DK rpers wonder why they never find RP and waste their time more on RP walking around than interacting with others.

[Deadraline]

I'd say they can smile. I just wouldn't be like Oh flowers and bunnies kind of happy though. But maybe smiling because they seen a good deed done? Or perhaps they had a good day and bought a cute new outfit from their local smith. Something more normal in the sense where most characters would be happy. I believe DK's can love, be sad, be happy, afraid even. But also remember to keep race in mind. Class is one thing, but don't go outside the boundaries of race either. Make sure however you make your character act if you take RP'ing in a serious sense of staying in character, that you go by not only the class, but the race.

[Marthenil]

Smile? hmmm probably not. Laugh? Hell yeah, especially when they have just grabbed the Grimoire of Necromancy part 1, wich is so rare, from the dead cold fingers of a Scourge necromancer. Hell, they do laugh. In a twisted, maniacal way.
Although I dont think you can laugh without smiling, as already has been posted.