Symbolism, Imagery, Allegory
Marcus spends a lot of time talking about humans' place in the universe and how each rational being is related to one another. He reminds himself time and again that a person who is true to his or her nature is by definition a social being.
When Marcus says you should be a social being, he not saying that you should be interesting at cocktail parties or that you should enjoy chatting up your colleagues at the water cooler. It means you understand your purpose is to know your place in the world and your duty the gods, and it means you understand you should always work for the common good.
Whew. That's a lot to keep in mind—and a lot of ways to go wrong. Marcus sees antisocial behavior all around him, and he often remarks on how this kind of behavior mars the fabric of life, and not just on a local level. Yes, we're talking about macrocosm again—and the gods are not happy:
The soul of a man harms itself, first and foremost, when it becomes (as far as it can) a separate growth, a sort of tumour on the universe: because to resent anything that happens is to separate oneself in revolt from Nature... (2.16)
Interestingly enough, Marcus talks about the damage here strictly in terms of the person who is perpetrating the wrong. It's his soul that suffers. But Marcus, by calling the rebellious man a "tumor," implies that the entire social fabric of the universe is at stake. It's a gross metaphor, but it's effective. There's no mistaking who's going to suffer in this scenario.
The emperor knows that in the end, the universe is gonna make it—the gods will see to that. The warning in this imagery is for the individual who kicks against fate and duty and separates him- or herself from the natural order of things. Because just like a tumor, he or she can and will be cut off to maintain the health of the Whole.