Plato believed that because we feel unhappy internally when we do something wrong, eudaimonia is the highest feeling of moral thought and behaviour where there is real happiness from within. Happiness, according to him, was about living in the pursuit of various virtues, central to flourishing.
Plato never mentioned the term eudaimonia, but his writings on the concept of courage, justice, wisdom and moderation point towards the same domain of wellbeing.