A passage from Seneca
Seneca, On the Happy Life 4
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A man thus grounded must, whether he wills or not, necessarily be attended by constant cheerfulness and a joy that is deep and issues from deep within, since he finds delight in his own resources.
Related passages
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Men seek retreats for themselves, houses in the country, sea-shores, and mountains... But this is altogether a mark of the most common sort of men, for it is in thy power whenever thou shalt choose to retire into thyself.
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At dawn, when you have trouble getting out of bed, tell yourself: 'I have to go to work — as a human being.'
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Take care not to be Caesarified, or dyed with purple; for it does happen. Keep thyself then simple, good, pure, serious, free from affectation.