Portia, in this scene, is rather despairing of ever marrying--even if she lives to be as old as Sibylla she'll die as chaste (pure and untouched) as Diana, the virgin goddess. She laments her inability to do her own choosing of a husband because of her father's posthumous and rather unconventional plan for selecting a groom--they must choose the correct casket full of coins in order to win her hand.
"O me, the word 'choose!' I may
neither choose whom I would nor refuse whom I
dislike; so is the will of a living daughter curbed
by the will of a dead father. Is it not hard,
Nerissa, that I cannot choose one nor refuse none?"
Nerissa's reply is fairly straightforward and simple. She clearly does not think it's a "hard" fate--though that's much easier for her to say since she doesn't have to live with the edict herself.
Nerissa believes Portia's father father was a good man who had a good plan, inspired by his impending death.
"...therefore the lottery,
that he hath devised in these three chests of gold,
silver and lead, whereof who chooses his meaning
chooses you, will, no doubt, never be chosen by any
rightly but one who shall rightly love."
Nerissa's view is rather fatalistic and simplistic. She assumes whoever chooses the right chest will, of course, also be the right man--one who will love her (and presumably will be loved by her in return). She goes on to ask Portia how she feels about each of the proposed suitors, but Nerissa is confident in her romanticized view that all will end as it should--happily ever after.