Portia's father was obviously well-aware of his daughter's stubbornness when he added a clause to his Will insisting that she abide by certain conditions of marriage. Portia is unimpressed but loyal to her father and therefore, despite disliking her suitors, she is going ahead with his wishes.
In order to be sure that any suitors were indeed serious, the ramifications for anyone who failed to choose the correct casket which provided for Portia's hand in marriage were dire. The Prince of Arragon deliberated over the choices and considering himself superior to "many" chose the silver casket.
With one fool's head I came to woo, / But I go away with two" (II.ix.75-76).
Having made the wrong choice, he now has to leave immediately and to
never reveal their choice, never propose marriage to another maid, ..
Basically, his future is quite bleak.