Seriously, you people who say you haven't seen him smile, in the anime or the manga, don't you notice the scenes in which he smiles?

(I couldn't resist, I put this as my avatar.)
Generally, he has a calming, pure, adorable and heartwarming smile.

Except when he smiles somehow "evil" as in the pictures I posted further up. I think he does those "evil" smiles because he's starting losing his sanity. I believe he will not lose it though. He has Ichiru after all.

And Yuuki, in a way.
Ah, and there is a picture in the manga that shows indirectly that Zero is laughing.
Here:
May I add this? I forgot to post it.
Okay, seriously now, I'm sorry for this enormous post, but I just found another page in which Zero smiles. I'll upload the whole of it just because Zero is so sweet towards the little kid. If he's so sweet towards a strange kid, think about how sweet he would be if he had his own child (hopefully with Yuuki)

He's just so adorable!