Wretchedness is caused by emotional disturbances, and the happy life by calmness, and disturbance takes two forms — anxiety and fear in expecting evils, ecstatic joy and lustful thoughts in misunderstanding good things, all of which are at variance with with wisdom and reason. Accordingly, if a man possesses self-control and consistency, and is without fear, distress, excitability, or lust, is he not happy? But this is the nature of the wise man always, so he is happy always.

[Atque cum perturbationes animi miseriam, sedationes autem vitam efficiant beatam, duplexque ratio perturbationis sit, quod aegritudo et metus in malis opinatis, in bonorum autem errore laetitia gestiens libidoque versetur, quae omnia cum consilio et ratione pugnent, his tu tam gravibus concitationibus tamque ipsis inter se dissentientibus atque distractis quem vacuum solutum liberum videris, hunc dubitabis beatum dicere? atqui sapiens semper ita adfectus est; semper igitur sapiens beatus est.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher

Tusculan Disputations [Tusculanae Disputationes], Book 5, ch. 15 (5.15) / sec. 43 (45 BC) [tr. Davie (2017)]
    (Source)

(Source (Latin)). Alternate translations:

Now since the Disturbances of the Soul render the Life miserable, but the composure of them happy; and there is a double rank of Passions; in that, Discontent and Fear are terminated on Evils conceiv'd; but excessive Mirth and Lust arise from the misapprehension of good things, since all are inconsistent with Advice and Reason, if you shall see any one clear, emancipated, free from these emotions so vehement, so discordant one with the other, and so distracting, can you make any question of calling him Happy? But the Wise man is always so dispos'd, therefore the Wise man is always Happy.
[tr. Wase (1643)]

But as the perturbations of the mind make life miserable, and tranquility renders it happy: and as these perturbations are of two sorts; grief and fear, proceeding from imagined evils, immoderate joy and lust, from the mistake of what is good; and all these are in opposition to reason and counsel; when you see a man at ease, quite free and disengaged from such troublesome commotions, which are so much at variance with one another, can you hesitate to pronounce such a one a happy man? Now the wise man is always in such a disposition: therefore the wise man is always happy.
[tr. Main (1824)]

But when the perturbations render life unhappy, while their repose makes it happy -- and since the mode of perturbation is twofold -- sorrow and fear having birth from reputed evils -- the delirium of joy and desire, from the delusion of good, -- when all these are repugnant to counsel and reason, and you see a man void, exempt, free from these excitements, so vehement, so discordant, so distracted by mutual conflicts, -- will you hesitate to pronounce him happy? But the wise man is always thus, and therefore always happy.
[tr. Otis (1839)]

But as the perturbations of the mind make life miserable, and tranquillity renders it happy; and as these perturbations are of two sorts, grief and fear, proceeding from imagined evils, and as immoderate joy and lust arise from a mistake about what is good, and as all these feelings are in opposition to reason and counsel; when you see a man at ease, quite free and disengaged from such troublesome commotions, which are so much at variance with one another can you hesitate to pronounce such an one a happy man? Now the wise man is always in such a disposition, therefore the wise man is always happy.
[tr. Yonge (1853)]

Now since perturbations of mind create misery, while quietness of mind makes life happy, and since there are two kinds of perturbations, grief and fear having their scope in imagined evils, inordinate joy and desire in mistaken notions of the good, all being repugnant to wise counsel and reason, will you hesitate to call him happy whom you see relieved, released, free from these excitements so oppressive, and so at variance and divided among themselves? Indeed one thus disposed is always happy. Therefore the wise man is always happy.
[tr. Peabody (1886)]