@maxwellatoms Here's an in-depth analysis as to why curlines almost never smile.
Smiling is a very rare phenomenon that most curlines experience at least once in their lifetime. About half of them smile multiple times sporadically throughout their entire lives. It usually takes a very important or dramatic event for them to even crack the slightest of smiles.
A female curline by the name of Mandy has been observed smiling in video footage before. All of which was filmed when she was disguised as a little human girl. Most people have only watched a handful of the footage caught of her and never saw her smiling once. Obviously, this footage was from "Grim & Evil" and "The Grim Adventures of Billy & Mandy".
There was one occasion where someone edited existing footage as an homage to "2001: A Space Odyssey", where they made it look like her smiling for the very first time in her entire life broke the fabric of reality and put them in a completely different dimension reincarnated as completely different characters. This person obviously did not watch all of the footage of Mandy and assumed that she never smiled in her entire life before, even though she did.
A natural response to happiness for most curlines is a neutral look, with a wagging tail similar to a dog's, which is the equivalent of a human smile. While curlines have the facial muscles in their faces to smile, it generally takes quite a bit of effort to force one. There are curlines that do smile when they're happy such as Caleb, but it is pretty rare to find a curline who emotes happily like a human.
The first documented case of the curline named Mandy smiling was when she beat the grim reaper at a game of limbo. It was a very dramatic event that filled her with joy. This instance of her smiling was after the grim reaper's skull said, "I loathe you" to her as she was laying in bed reading a book.
Most people didn't watch the parts of the footage where Mandy smiled sporadically, hence why they claimed that she never actually smiles. The human boy named Billy that she hung out with almost never saw her smile and also claimed that she never did.
So all-in-all, not every intelligent creature expresses its joy through smiling or laughter. A curline smiling may not actually indicate happiness sometimes, as rare as it is. Much like a hyena laughing doesn't mean it's actually laughing or finds something funny. It's most likely expressing frustration or anger.