Don't let the impression of other people's grief carry you away indiscriminately. Help them, yes, as best you can and as the case deserves, even if their grief is for the loss of something indifferent: but do not imagine their loss as any real harm—that is the wrong way of thinking. (5.36)
Marcus voices a central tenet of Stoicism: keep the drama to a minimum. While empathy is important—especially when trying to figure out a person's motivations—it can go too far. It's never a good idea for you to indulge your emotions too far, whether in grief or happiness. Manic highs and lows, for Marcus, are a sign that you're not in control of your mind and are too susceptible to sense impressions. The further you can distance yourself from emotional response, the more divine you are.
So display those virtues which are wholly in your own power—integrity, dignity, hard work, self-denial, contentment, frugality, kindness, independence, simplicity, discretion, magnanimity. Do you see how many virtues you can already display without any excuse of lack of talent or aptitude? (5.5)
Marcus is feeling a little sorry for himself, since he wasn't born with any obvious talent. Essentially, he tells himself to quit whining, since he has virtues enough—if only he would display them. In the end, it's a failing of his own that he doesn't appear as accomplished as he is (he's emperor, after all). If he can follow his fine principles, someone will surely notice.