Portia, like Antonio, in "The Merchant of Venice" is "weary" of the world (Act 1, Scene 2, lines 1-2). Why? Is her weariness anything like Antonio's?
Good close reading! Yes, it's no accident that the two first scenes of "The Merchant of Venice" start with characters professing their weariness, and one of the effects this creates is to set up a parallel between Antonio and Portia, both of whom end up competing for Bassanio's attentions.
Antonio is usually considered to be "sad" because of his (perhaps undeclared) love for Bassanio; though Portia, on the other hand, as Nerissa points out, is not sad because she lacks anything, but because she's got everything she could possibly want. It's a sort of upper-class boredom:
PORTIA: By my troth, Nerissa, my little body is a-weary of
this great world.
NERISSA: You would be, sweet madam, if your miseries were
in the same abundance as your good fortunes are; and yet, for aught I see, they are as sick that surfeit with too much, as they that starve with nothing.
Perhaps she is fed up with the silly game her father's will insist she plays to find a suitor, perhaps she is just fed up - but the joky tenor of the rest of the scene suggests that there isn't anything very much wrong. Portia - a rather unpleasant racist and a spoilt little rich girl - after all, has nothing very much to complain about.