Why does Marcus Aurelius say we should trust the governing reason of the universe?

Marcus does not ask you to trust the universe on faith. He argues for it. In Meditations 5.32 he defines the kind of soul that has genuine skill and knowledge: it is the one that knows beginning and end, and knows the reason which pervades all substance and through all time by fixed periods administers the universe. His claim is that the cosmos is not run by accident or by a blind force but by a reason (logos) that understands its own workings completely, the way a craftsman understands a craft. Trust, for Marcus, follows from that understanding, not from wishful thinking.

The common misreading treats this as resignation: a tired man telling himself things will be fine because there is no point resisting fate. That reading gets the argument backwards. Marcus is not saying trust the universe because you cannot change it. He is saying trust it because it has skill and knowledge that you can reason your way to recognizing, in the same way you would trust a skilled physician over an unskilled one. The disturbance in 5.32 comes from unskilled and ignorant souls trying to unsettle someone who actually has knowledge. Marcus places the ordering reason of the universe in that same category of genuine expertise, and contrasts it with human confusion.

He makes the reasoning explicit elsewhere in the same book. In Meditations 2.11 he lays out the structure directly. If the gods exist and they care about human affairs, he writes, they have put all the means in man's power to enable him not to fall into real evils. From there he draws out what that would mean for a governing reason to fail: it would have to be through ignorance, or through knowledge without the power to guard against harm, or through the sheer mistake of letting good and evil fall on people at random. Marcus considers each option and rejects them all as beneath a reason that governs by fixed periods rather than by chaos. A universe administered by real knowledge does not make basic errors of that kind.

That is why he can then say that death, life, honor, dishonor, pain and pleasure fall equally on good people and bad people. If the governing reason were careless or incompetent, that indiscriminate distribution would be a scandal. But because Marcus trusts that reason is neither ignorant nor powerless, he concludes instead that these things must not be the real goods and evils at all. They are external to the one thing a rational being actually controls: the quality of its own judgments and choices. Trusting the universe's reason and holding that only character is good or bad turn out to be the same conviction stated twice.

What changes for someone who takes this seriously is where they look for competence. You stop treating a setback, a diagnosis, or an unequal outcome as proof that the world is broken or unfair, because you no longer expect the world to hand out comfort as evidence of its rationality. You expect it to be ordered, not to be gentle. That is a harder standard to live by than it sounds, and Marcus is not offering it as comfort. He is offering it as the conclusion of an argument: a whole that knows its own beginning and end has no reason to bungle the distribution of pain and pleasure, so bungling is not what is happening when you suffer.

One way to use this: the next time something falls on you that feels arbitrary, ask what in it is actually up to you, your judgment and your response, and set the rest down as neither good nor bad, exactly as Marcus does with death and dishonor.