Man’s soul harms itself, firstly and chiefly when it does all it can to become a seperate growth, a sort of tumor on the Universe. To resent any particular event is to revolt against the general law of Nature, which comprehends the order of all events whatsoever. It also dishonors the soul when it has aversion to any man, and opposes him with intention to hurt him, as wrathful men do. Thirdly, it affronts itself when conquered by pleasure or pain; fourthly, when it does or says anything hypocritically, feignedly or falsely; fifthly, when it does not direct to some proper end all its desires and actions, but exerts them inconsiderately and without understanding. For, even the smallest things should be referred to the end, and the end of rational beings is to follow the order and law of the venerable state and polity which comprehends them all.
A soul “affronts itself when conquered by pleasure or pain”.
This is perhaps the essence of stoicism in a single line. The goal of stoicism is not to feel nothing, but rather to have control over those feelings. Yes, we feel love, or anger, but letting yourself be control by emotional impulses harms oneself. Perhaps internalizing this one line is internalizing the central lesson of Marcus.