Full Idea
Contentment [euthumia] comes about for human beings from the moderation of enjoyment and proportion [summetria] in life.
Gist of Idea
Contentment comes from moderation and proportion in life
Source
report of Democritus (fragments/reports [c.431 BCE], B191) by John Stobaeus - Anthology 3.1.210
Book Reference
Democritus: 'Early Greek Phil VII: Democritus', ed/tr. Laks,A/Most,G [Harvard Loeb 2016], p.261
A Reaction
This is close to Aristotle's doctrine of the Mean. The majority of ethical ideas attributed to Democritus (presumably by the Epicureans) are thought to be spurious. This idea actually sounds rather stoic.