Letters 81.21

Thus, as I have said, your being grateful is more conducive to your own good than to your neighbour’s good. For while your neighbour has had a common, everyday experience,—namely, receiving back the gift which he had bestowed,—you have had a great experience which is the outcome of an utterly happy condition of soul,—to have felt gratitude. For if wickedness makes men unhappy and virtue makes men blest, and if it is a virtue to be grateful, then the return which you have made is only the customary thing, but the thing to which you have attained is priceless,—the consciousness of gratitude, which comes only to the soul that is divine and blessed. The opposite feeling to this, however, is immediately attended by the greatest unhappiness; no man, if he be ungrateful, will be unhappy in the future. I allow him no day of grace; he is unhappy forthwith.

Seneca, Letters to Lucilius 81.21, translated by Richard Mott Gummere