Not observing what is in the mind of another a man has seldom been seen to be unhappy; but those who do not observe the movements of their own minds must of necessity be unhappy.
Others are unknowable, you know this. It should not sadden you. But not knowing yourself? That’s a problem.
Like many of the meditations, its easy to read this as a call to meditation. Meditation, as we conceive it, would likely have been foreign to Aurelius, but a certain type of mindfulness? Of self-knowledge? I think that is what he is getting at here.