Commentary on Meditations: B5:36-37

luck is the good fortune you determine for yourself

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. Rather, you should be like the old man in the play who reclaimed at the end his foster-child's favourite toy, never forgetting that it was only a toy. So there you are, broadcasting your pity on the hustings - have you forgotten, man, what these things are worth? 'Yes, but they are important to these folk.' Is that any reason for you to join their folly?

'There was a time when I met luck at every turn.' But luck is the good fortune you determine for yourself: and good fortune consists in good inclinations of the soul, good impulses, good actions.

In the first passage, Marcus tackles dealing with impressions other peoples' grief may cause.  In a word - don't let others' grief spill into your impressions.  You know better.  However, that does not mean you have to treat others coldly.  You can have empathy and allow the grieving person to grieve over loss of indifferents.  But you don't have to go so far as agree with the negative assents (that the loss is bad) or emotions or even that these things truly matter to a Stoic.

The second passage reminds me of Harvey Dent (from Batman), who has a double-headed coin.  No matter which side lands, he wins.  He makes his own luck - it's all about perception.  Marcus, more appropriately, states good luck comes from a good attitude, good desires and good actions.

(see also Citadel p. 217)