Disclaimer: I don't own SPN.
The basic premise behind Cas and his lack of smiling was borrowed from 'A Room of One's Own' by NorthernSparrow on AO3. That fic is absolutely BRILLIANT and makes a very valid point in that angels are an entirely different species from human, rather than just superpowered humans, and therefore they have their own mannerisms and everything. I didn't go THAT far but it was definitely the inspiration behind this.
Thanks to mollienaturerocks, RangerHorseTug, Bipi and masterjediratgrl31 for your reviews :)
76. Smile
One of Dean's absolute favourite sights was Cas' smile. As an angel, Cas hadn't smiled very much. He'd explained at some point that this was because he was not actually human but an angel inhabiting a human body; a whole other species. This meant that he didn't rely so much on his vessel to convey his emotions – because apparently angels did feel some sort of emotions – but instead expressed them through certain angelic mannerisms that were easy for humans to miss (mostly because they were expressed through his true form, which humans couldn't perceive).
So when Cas did physically smile, Dean felt a spark of accomplishment surge through him. Making Cas smile through Jimmy Novak's face meant that he was pretty darn happy and Dean took great pride in the fact that he was responsible for nearly all of those smiles.
But now that Cas was human, Dean didn't know whether to feel happy or disappointed. On the one hand, now that Cas expressed his emotions outwardly, it took much less to make him smile and therefore Dean felt less accomplished – as though he'd turned down the difficulty in a video game. But on the plus side, it meant that he got to revel in that beautiful smile, those crinkled blue eyes and that low, deep laugh more often. And that couldn't be a bad thing, could it?