Before you were born, people were "working for your benefit." Have you ever contemplated that idea? We are born and injected with the philosophy (for better or for worse) of our parents and those who help to raise us. We have no say in the matter of how they were raised philosophically. For all we know, their philosophy in which they were raised or came to believe, is misguided and focused on the wrong things. Some people are fortunate enough to have parents come to a correct understanding of what is good, and subsequently teach their children. The rest of us simply stumble along and grasp at straws in the dark.
But perhaps the answer to this problem can be found in the writings and ideas of those who have spent a lifetime thinking, discussing and debating this important question. This is what Seneca means, I think. If a person's soul becomes "roused" enough to seek for what is the good, then there are opportunities to learn.
Pick up the list of the philosophers; that very act will compel you to wake up, when you see how many men have been working for your benefit. You will desire eagerly to be one of them yourself. For this is the most excellent quality that the noble soul has within itself, that it can be roused to honourable things.
No man of exalted gifts is pleased with that which is low and mean; the vision of great achievement summons him and uplifts him.
But happy is the man who has given it this impulse toward better things! He will place himself beyond the jurisdiction of chance; he will wisely control prosperity; he will lessen adversity, and will despise what others hold in admiration.
There is the answer - begin to learn from the philosophers! Start reading and contemplating their writings. Talk to others about it. Write about it; then repeat. And once you begin to learn, you will also learn to live wisely.
You begin to learn what is important; what belongs to you and what does not. You will learn learn what virtue and vice really mean; what has utility and what does not.
This last section of his letter, to me, sounds a bit more Epicurean than Stoic.
Utility measures our needs; but by what standard can you check the superfluous? It is for this reason that men sink themselves in pleasures, and they cannot do without them when once they have become accustomed to them, and for this reason they are most wretched, because they have reached such a pass that what was once superfluous to them has become indispensable. And so they are the slaves of their pleasures instead of enjoying them; they even love their own ills, – and that is the worst ill of all! Then it is that the height of unhappiness is reached, when men are not only attracted, but even pleased, by shameful things, and when there is no longer any room for a cure, now that those things which once were vices have become habits.