Every living organism is fulfilled when it follows the right path for its own nature. For a rational nature the right path is to withhold assent to anything false or obscure in the impressions made on its mind, to direct its impulses solely to social action, to reserve its desires and aversions to what lies in our power, and to welcome all that is assigned to it by universal nature. Because it is a part of universal nature just as the nature of the leaf is part of the plant's nature: except that in the case of the leaf its nature partakes of a nature which lacks perception or reason and is liable to impediment. Whereas man's nature is part of a nature which is unimpeded, intelligent, and just - in that to each creature it gives fair and appropriate allocations of duration, substance, cause, activity, and experience. But do not look to find a one-to-one correspondence in every case, but rather an overall equivalence - the totality of this to the aggregate of that.
Not possible to study. But possible to rein in arrogance; possible to triumph over pleasures and pains; possible to rise above mere glory; possible not to be angry with the unfeeling and the ungrateful, and even, yes, to care for them.
Let nobody any more hear you blaming palace life: don't hear yourself blaming it.
The nature of humans is to be (think and act) rational. We are rational when we exercise the discipline of assent. What does this mean? We will receive impressions from our environment and other people. We don't have to constantly react without thinking. We can identify things that are in our control and out of our control. Once we've made that identification, we can then base our contentedness on things that are in our control. Otherwise, if we think we will be happy by seeking for something out of our control, we put ourselves at risk of being sad and depressed. Therefore, we can do as Marcus counsels himself - we can withhold our assent (agreement) to anything false or obscure (vices and things out of our control).
Furthermore, as rational creatures, we can accept Universal nature - or those things that happen in the world and universe. We can't control them, but we have to accept and live with them. Death is a great example of something that is universal and it is something we all have to accept.
Humans can "triumph over" things out of our control. We can extend our desires to living virtuously as opposed to living for pleasure or striving for pain avoidance. It is all about pointing our desires to virtue. It is possible to not want fame. It is possible not to be angry with mean people. It is even possible to care for the mean-spirited people. Jesus said this with some very powerful words: "turn the other cheek."
In the last passage, Marcus chides himself to not complain about palace life.
(see also Citadel p. 29, 44, 70, 138, 219, 221, 241, 269-270, 286, 291)