I'd submit there's a pretty simple explanation - we've seen (and heard) all year about KG's discipline, about how hard he is on himself and how seriously he's taking this run at a championship, about how Pierce is dancing in the locker room 60 minutes before a game while KG is locked in from his first p--- in the morning.
This run matters to KG, a lot. Not necessarily more or less than to Pierce or Allen or Rivers, etc., but, when something matters that much to you, in tends to amplify and exaggerate your personality and instincts. For Pierce, that's to be a joker and ecstatic and stay loose. For KG, that's to lock-in and focus until his bowels seize up.
When this is over, and the Celtics have inevitably prevailed, KG is going to exhale so loud the moon will be blown off its axis. Until then he's consumed by the ultimate goal. I don't think it's that he didn't want to smile last night, it's that he couldn't allow himself to (even though I suspect he was desperate to let it out).
It may be difficult to relate to, but it's the essence of KG.