In The Merchant of Venice, Portia is an independent young woman but her loyalty to her father is indisputable, despite her dislike of the conditions placed upon her by his last will and testament as "the will of a living daughter (is) curb'd by the will of a dead father."(I.ii.24-25) Portia, however, dislikes all her potential suitors except Bassanio and tries to manipulate the situation in her favor. When it is Bassanio's turn, she even tries to "eke it" out and tell him to wait a while to make his decision in case he chooses incorrectly.
Portia, after Bassanio chooses correctly, humbles herself admitting that she is "unschooled, unpractised" (III.ii.160) which the audience can understand as, in Shakespeare's day, the woman would have been subservient and, for all her fighting spirit, she is still obedient and recognizes the boundaries of her time. At the same time, she quietly and subtly manipulates many situations, unnoticed.
By giving the appearance of the perfect future wife, Portia is able to set her own conditions, particularly with the ring which Bassanio must treasure; otherwise, "Let it presage the ruin of your love." It is Portia's way of maintaining control without embarrassing Bassanio or exposing herself.