"In the third century B.C., the Greek stoic philosopher Chrysippus died of laughter after giving his donkey wine, then seeing it attempt to feed on figs"
He looks like the most serious Greek I have seen! I can't imagine him, a respected philosopher, paving the way for all future thought, would get a donkey drunk and watch him eat figs. And then find it funny enough to die.
Maybe the sculptor had just met him on a bad day. Or maybe he was jokingly pulling his 'serious look', while bursting into laughter every few seconds. Looks like you can't judge a bust by its cover*.
*I know that doesn't make sense. I was just trying to be clever.