I know it's not the actual reason but this is very unatrual

He’s always grinning because he’s a skeleton and he has no lips

From what I've heard mainly is a poker face or obscuring his feeling, in the last breath au it shows he's capable of frowning in the end and 2nd phase

It doesn't look THAT bad

Correct but it's a little something called ExAgGEraTioN

see, he smiles because he has no lips

That's probably true but I've heard other reasons. And you kinda need lips to pull a smile, because otherwise he'd just be doing some sorta face like a dog showing it's teeth. I'm not trying to start an argument if you think that is so, I'm just sharing my side.

Only time his mouth changes is when you hit him, and even then he still has a very noticable smile

Yeah. Tbh him keeping his smile is kinda admirable

reminds me of the story of undertale sans tbh

Truthfully I've never played undertale buti know a lot about the au games