The source of the difficulty is that the translation in the question is not quite right. From most important to least:
- The translation has no equivalent for “bonum” meaning “the abstract idea of goodness in Platonic philosophy”, which is the very subject that Seneca is discussing in the letter, and so the most important word in the sentence!
- Seneca’s sentence begins “Ex his ergo” meaning “out of these therefore”, which has been lost somehow.
- The translation renders “cura” as “care” but to a modern reader that fails to convey Seneca’s intended sense, which is more like “work, effort, diligence, pains”.
- The translation renders “perfecit” as “is fulfilled by” but I think Seneca’s intended sense is more like “perfects, completes”.
- The translation swaps active for passive in each clause: Seneca says that Nature perfects the goodness of a god, not that a god is perfected by Nature.
- The translation omits “scilicet” meaning “namely, specifically”.
I would translate the sentence as:
Therefore Nature perfects the goodness of one of these, namely the god, and work that of the other, namely man.
The Loeb translation is closer to the original but rather unwieldy as an English sentence:
Of one of these, then—to wit God—it is Nature that perfects the Good; of the other—to wit man—pains and study do so.
Seneca the Younger. Epistle 124. Translated by Richard M. Gummere (1925). Seneca ad Lucilium epistulae morales, volume 3, p. 445. London: William Heinemann.
So Seneca in this passage is contrasting the difference between the goodness of a god and the goodness of a person: the former comes naturally but the latter requires effort. The letter as a whole is discussing the nature of goodness: apparently Seneca’s correspondent Lucilius had equated goodness with pleasure, and Seneca replies that Stoicism holds otherwise:
Those who rate pleasure as the supreme ideal hold that the Good is a matter of the senses; but we Stoics maintain that it is a matter of the understanding, and we assign it to the mind. If the senses were to pass judgment on what is good, we should never reject any pleasure; for there is no pleasure that does not attract, no pleasure that does not please. Conversely, we should undergo no pain voluntarily; for there is no pain that does not clash with the senses. Besides, those who are too fond of pleasure and those who fear pain to the greatest degree would in that case not deserve reproof. But we condemn men who are slaves to their appetites and their lusts, and we scorn men who, through fear of pain, will dare no manly deed. But what wrong could such men be committing if they looked merely to the senses as arbiters of good and evil? For it is to the senses that you and yours have entrusted the test of things to be sought and things to be avoided!
Seneca, p. 437.
This leads Seneca to a discussion of goodness generally, and eventually to the different kinds of goodness in the different orders of being:
There are four natures which we should mention here: of the tree, animal, man, and God. The last two, having reasoning power, are of the same nature, distinct only by virtue of the immortality of the one and the mortality of the other. Of one of these, then—to wit God.—it is Nature that perfects the Good; of the other—to wit man—pains and study do so. All other things are perfect only in their particular nature, and not truly perfect, since they lack reason.
Seneca, p. 445.
Seneca says that the goodness of a god comes naturally because the god is immortal, and the goodness of people requires effort because people are mortal. Seneca doesn’t give an explanation of how this works, but the ideas Seneca is expounding in this letter mostly originate with Plato’s Republic, and so perhaps the idea is that immortal things are unchanging (and so can only be good or bad by their nature), and mortal things are variable (and so can struggle and improve themselves), an idea which appears in Plato, for example:
Which has a more pure being—that which is concerned with the invariable, the immortal, and the true, and is of such a nature, and is found in such natures; or that which is concerned with and found in the variable and mortal, and is itself variable and mortal?
Plato (4th century BCE). The Republic, 9.585c. Translated by Benjamin Jowett (1888). The Republic of Plato, p. 298. Oxford: Clarendon Press.