At Belmont, what reason does Portia give Nerissa for being weary of the world?

Great question. Some readers suggest that Portia is weary of the world because life has been too easy for her. She is weary of having so much luxury and ease. In act 1, scene 2, Nerissa points out the easy living, wealth, and good fortunes that Portia has had throughout her life. Portia then elaborates on what is bothering her:

Portia: . . . 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?

In this passage, Portia explains why she is weary of her father's plans for her future marriage. Though her father is dead, he left specific rules in his will about who she can marry. Anyone who wants to marry her has to go along with her father's game. This game involves suitors selecting one of three caskets, gold, silver, and lead. Only one casket allows them the right to marry Portia. Additionally, suitors must agree not to marry anyone else, even if they choose the wrong casket. Though many men want to marry Portia, Portia has no right to choose a husband for herself. Additionally, she doesn't find many of the suitors particularly interesting.
When Portia tells Nerissa that she is weary with the world, she means that she is bored by her father's marriage game. Portia has no power in choosing her own husband; instead, she has to wait for a suitor to win her father's game. She is bored of waiting for her future husband to arrive, and she worries that she may not like the man who eventually wins the right to marry her.