Democritus, fragment 191 (tr. Kathleen Freeman):
Cheerfulness is created for men through moderation of enjoyment and harmoniousness of life. Things that are in excess or lacking are apt to change and cause great disturbance in the soul.
ἀνθρώποισι γὰρ εὐθυμίη γίνεται μετριότητι τέρψιος καὶ βίου συμμετρίῃ· τὰ δ᾿ ἐλλείποντα καὶ ὑπερβάλλοντα μεταπίπτειν τε φιλεῖ καὶ μεγάλας κινήσιας ἐμποιεῖν τῇ ψυχῃ.
Souls which are stirred by great divergences are neither stable nor cheerful.
αἱ δ᾿ ἐκ μεγάλων διαστημάτων κινούμεναι τῶν ψυχέων οὔτε εὐσταθέες εἰσὶν οὔτε εὔθυμοι.
Therefore one must keep one's mind on what is attainable, and be content with what one has, paying little heed to things envied and admired, and not dwelling on them in one's mind. Rather must you consider the lives of those in distress, reflecting on their intense sufferings, in order that your own possessions and condition may seem great and enviable, and you may, by ceasing to desire more, cease to suffer in your soul.
ἐπὶ τοῖς δυνατοῖς οὖν δεῖ ἔχειν τὴν γνώμην καὶ τοῖς παρεοῦσιν ἀρκέεσθαι τῶν μὲν ζηλουμένων καὶ θαυμαζομένων ὀλίγην μνήμην ἔχοντα καὶ τῇ διανοίᾳ μὴ προσεδρεύοντα, τῶν δὲ ταλαιπωρεόντων τοὺς βίους θεωρέειν, ἐννοούμενον ἃ πάσχουσι κάρτα, ὅκως ἂν τὰ παρεόντα σοι καὶ ὑπάρχοντα μεγάλα καὶ ζηλωτὰ φαίνηται, καὶ μηκέτι πλειόνων ἐπιθυμέοντι συμβαίνῃ κακοπαθεῖν τῇ ψυχῇ.
For he who admires those who have, and who are called happy by other mortals, and who dwells on them in his mind every hour, is constantly impelled to undertake something new and to run the risk, through his desire, of doing something irretrievable among those things which the laws prohibit.
ὁ γὰρ θαυμάζων τοὺς ἔχοντας καὶ μακαριζομένους ὑπὸ τῶν ἄλλων ἀνθρώπων καὶ τῇ μνήμῃ πᾶσαν ὥραν προσεδρεύων ἀεὶ ἐπικαινουργεῖν ἀναγκάζεται καὶ ἐπιβάλλεσθαι δι' ἐπιθυμίην τοῦ τι πρήσσειν ἀνήκεστον ὧν νόμοι κωλύουσιν.
Hence one must not seek the latter, but must be content with the former, comparing one's own life with that of those in worse cases, and must consider oneself fortunate, reflecting on their sufferings, in being so much better off than they.
διόπερ τὰ μὲν μὴ δίζεσθαι χρεών, ἐπὶ δὲ τοῖς εὐθυμέεσθαι χρεών, παραβάλλοντα τὸν ἑαυτοῦ βίον πρὸς τὸν τῶν φαυλότερον πρησσόντων καὶ μακαρίζειν ἑωυτὸν ἐνθυμεύμενον ἃ πάσχουσιν, ὁκόσῳ αὐτέων βέλτιον πρήσσει τε καὶ διάγει.
If you keep to this way of thinking, you will live more serenely, and will expel those not-negligible curses in life, envy, jealousy and spite.
ταύτης γὰρ ἐχόμενος τῆς γνώμης εὐθυμότερόν τε διάξεις καὶ οὐκ ὀλίγας κῆρας ἐν τῷ βίῳ διώσεαι, φθόνον καὶ ζῆλον καὶ δυσμενίην.
Democritus, fragment 200 (tr. Kathleen Freeman):
People are fools who live without enjoyment of life.
ἀνοήμονες βιοῦσιν οὐ τερπόμενοι βιοτῇ.