Why is Portia angry with her deceased father?

When the audience first meets Portia, she is lamenting her difficult situation to Nerissa at her palace in Belmont. Portia expresses her displeasure by telling Nerissa,

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? (Shakespeare, 1.2.21–24)

Portia is upset that she cannot choose the man she will marry. Portia's deceased father created a lottery, where potential suitors were given the opportunity to choose from one of three caskets with the hope of winning Portia's hand in marriage. Portia has no control over her marriage, and her future is completely dependent on the person who selects the correct casket. To make the situation worse, Portia is not attracted to any of the potential suitors, who have expressed interest in participating in the lottery to win her hand in marriage. As Nerissa lists the names of potential suitors, Portia dismisses each one of them by highlighting their negative personality traits. Portia desires to marry Bassanio but cannot do so unless he chooses the correct casket.

Portia wishes her deceased father had left her free to choose her own husband. Instead, in his will, her father stipulated that Portia's husband be chosen by a "lottery." The man who wins her hand in marriage must choose correctly between three caskets: one gold, one silver, and one lead. The man who picks the right casket is the man she must marry, whether she wants to do so or not. She can not choose her mate, nor can she refuse the one who selects the appropriate casket.
Portia doesn't think much of the men competing for her hand as the play opens. We see her early on as an assertive woman with a mind of her own. She would like to be allowed to make her own decision in a matter as important as choosing her husband. As she says to her maid, Nerissa:

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?