Marcus Aurelius was steeped in the thoughts of the Greek and Roman stoics who, starting with Zeno, focused on the search for a firm support for the moral life. "How should I live?" was the great and overriding question for them. Following on from Zeno, Epictetus, and Seneca, Marcus Aurelius portrayed in his Meditations the idea that the importance of philosophical inquiry lay in its significance for the moral life. He said, “Always think of the universe as one living organism with a single substance and a single soul.” This leads to the basic Stoic perception that “there is a law which governs the course of nature and should govern human actions.”(Meditations, p 73)
Marcus Aurelius emphasizes several other themes in his notes on life known as the Meditations. Among them are the tenets that underlie the stoic philosophy that he learned from his teachers including a discussion of the importance of your duty both to your own nature and that of the whole universe. It is with these tenets in mind that we see him telling us to accept what is beyond our control (5.8) in his expression of the notion that freedom for man is possible only when he is indifferent to the his fate as decreed by nature. This is consistent with the view of Epictetus in his Enchiridion. Both emphasize that this in the sense that the we are all a part of the whole of nature and recognition of that is necessary to achieve the good. The good which is always the moral good.
The importance of this is seldom clearer than when Aurelius notes the importance of focusing on the present, the "task at hand" if you will by exercising dispassionate justice in the following way:
"Vacating your mind from all its other thoughts. And you will achieve this vacation if you perform each action as if it were the last of your life: freed, that is, from all lack of aim, from all passion-led deviation from the ordinance of reason, from pretense, from love of self, from dissatisfaction with what fate has dealt you." (2.5)
It is acting like this, not in any morbid sense, but with a cheerfulness of mind, as described in the quote from Seneca above, that you will achieve the tranquility of being that is the ultimate form of happiness. But there is more than happiness in Stoicism and honestly that is not the primary goal of the stoic life.