|luck is the good fortune you determine for yourself|
'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)