"This is the thought that you should keep at hand to apply whenever you lose any external thing: what are you acquiring in exchange for it?" (v. 1, p. 238)
The way he stats this is a bit cryptic, but further on, he makes that point that you wouldn't feel you've lost out if you exchanged a donkey in return for a horse or a sheep in return for an ox. And when applied to losing external things (things outside our control), you would be worse off if you got upset by this loss, while on the other hand, if you keep your cool, you will have gained in virtue (wisdom, courage, self-control, justice).
And if he wasn't clear enough in this point, he makes it clearer when he says, "if you nod off just for a moment, all that you've amassed up until then, is lost and gone. Pay careful attention, then, to your impressions; watch over them unceasingly. For it is not something of little importance that you're trying to preserve, but self-respect, fidelity, impassibility, freedom from distress, fear, and anxiety, and in a word, freedom. At what price will you sell that? Consider how much it is worth." (v. 6-8, p. 238)
"Safeguard your own good in all that you do; and as for the rest, simply take what is granted to you in so far as you can make reasonable use of it, and be satisfied with that alone." (v. 11, p. 239)
Practically speaking, you have to do the math all the time. If you want the serenity and you want to maximize it, and when something out of your control happens or some external possession is lost to you, will being anxious or complaining about it fix the situation? Most likely not. And in fact, by giving in to those emotions, you begin to lose all momentum you've previously gained.