Commentary on Meditations: B6:45

All that happens to the individual is to the benefit of the Whole.  So far, so clear. But if you look more closely you will also see as a general rule that what benefits one person benefits other people too - though here 'benefit' should be taken in its popular application to things which are in fact indifferent.

Not much to discuss here.  This is Marcus saying nothing more than, "what is good for the goose, is good for the gander, but 'good' should mean 'indifferent.'"  This thought is somewhat synonymous with "do unto others what you would have them do unto you."  Obviously, it cannot be applied with 100% accuracy to every single person, but as a general rule of thumb, it is good advice to follow.