Maybe Democritus' tone would seem different to us if we had more of his work, or maybe this was an area (like determinism) where Epicurus was modifying what Democritus had taught.
You're right. Either is a possibility.
Was Democritus laughing because he was truly happy, or was his laughter cynical and to the effect that people are nothing but whirling windbags of atoms bouncing around with no more intelligence than billiard balls?
My take has always been that Democritus is laughing because he doesn't take himself too seriously, in the end we're all atoms and void. I think he can laugh about people who get caught up in the rat race (to use a modern metaphor) and take themselves too seriously. People - all things! - really are *ultimately* nothing more than "whirling windbags of atoms." That doesn't mean in any way that we don't enjoy our lives at the level we experience them! But chill out! Take a breath! Carpe diem - pluck the fruit of each moment.
PS. From Heraclitus 's Wikipedia article:
Weeping philosopher
In Lucian of Samosata's "Philosophies for Sale," Heraclitus is auctioned off as the "weeping philosopher" alongside Democritus, who is known as the "laughing philosopher" part of the weeping and laughing philosopher motif. This pairing, which may have originated with the Cynic philosopher Menippus, has been portrayed several times in renaissance art, where it generally references their reactions to the folly of mankind.[better source needed] Heraclitus also appears in Raphael's School of Athens.