At the beginning of the play, we learn that Bassanio is eagerly desirous of going to Belmont, Portia's home. In his conversation with Antonio in Act 1, Scene 1, he makes it clear that he wishes to woo her and win her hand in marriage. He describes her in glowing terms and says that she is more beautiful than beautiful and that she possesses "wondrous virtues." He also mentions that Portia has shown an interest in him.
It seems odd, though, that the first thing Bassanio mentions about Portia is that she is "richly left." He also focuses his conversation on her physical attributes and the fact that her value is known far and wide. He does not once mention how he actually feels about her. The impression is created that Bassanio's primary interest in her is linked to her enormous wealth. As the sole heir to her deceased father's fortune, Portia is exceedingly rich.
We also learn that Bassanio is a spendthrift who consistently borrows money that he hardly ever repays. He apologizes to Antonio for not having paid his bills and promises to repay all his outstanding debts, plus the one he is about to make if Antonio decides to assist him in his venture. He sweet-talks Antonio by telling him that he has to compete against many other wealthy suitors and wishes that he has the means to do so. At the end of the scene, the kindhearted and generous Antonio asks Bassanio to inquire from whom he can make a loan on the basis of a surety that he, Antonio, will provide.
Bassanio is clearly a wastrel who enjoys the good life without considering the consequences of his prodigality. He is described as a scholar and a soldier and cannot be earning much to support his flamboyant lifestyle. Marriage to Portia will provide him access to even greater indulgences and a better life than the privileged, albeit parasitic, one that he has been living so far. Bassanio's interest in Portia seems, especially in the early parts of the play, more economic than romantic.
Portia, in contrast, seems to be truly smitten with Bassanio. During her conversation with Nerissa in Act 1, Scene 2, she exposes her sentiments by agreeing with her that he is "best deserving a fair lady." Later, in Act 3, Scene 2, when talking to Bassanio, she, in convoluted language, essentially tells him that she is all his, and that even though she does not yet officially belong to him she actually does (in her heart). Bassanio expresses a similar sentiment about her. The difference between the two is that his declaration is not as direct as hers, and he uses a very roundabout way of expressing his affection.
When Bassanio chooses the right casket, Portia joyously pours out her commitment to him in a fairly long speech. She gives him a ring to seal their union and asks that he never remove it. Bassanio is somewhat overwhelmed by her outpouring and promises that only death would part him from her gift. Bassanio, in spite of what he says, does not, even at this juncture, sound entirely convincing.
Portia later displays her absolute commitment when she offers to pay all Antonio's debts. She is prepared to help out her fiance's best friend out of love for Bassanio. She later goes as far as disguising herself as a lawyer and defending Antonio against the malicious Shylock's claim for restitution. She essentially saves Antonio's life and humbles Shylock by cleverly using the law against him.
In the final scene of the play, Portia's farce about the ring that she had given Bassanio drives him almost to his wits' end. He apologizes profusely about having given the ring to the lawyer (Portia herself) who had saved his friend's life and begs her to understand. It becomes clear, once again, that Bassanio is prone to making promises he cannot keep. He has, essentially, pledged his life to retain the token of Portia's affection but gives it away at the first opportunity. He has been making similar promises to Antonio. Portia, however, relents when Antonio intervenes, and she forgives Bassanio his indiscretion and explains the whole story about her and Nerissa's disguises.
At the end of the play, one is still not quite sure about Bassanio's motives and whether he truly loves Portia or not. There is certainty about her, though, for she has shown her love and commitment both verbally and by her actions. He, on the other hand, never declares his love directly. All he ever does is suggest his affection through insinuation and innuendo.
When we encounter Portia we discover that she is a wealthy heiress on the cusp of inheriting her father's entire estate if she meets the terms of his will - that she should marry the suitor who chooses the right casket. She is constantly accompanied by Nerissa who is her lady-in-waiting and also her closest confidante.
The close personal attachment between the two women is clearly conveyed in the manner in which they communicate. Portia hardly sees Nerissa as a servant but deems her a friend. She divulges and shares all her thoughts, feelings, desires and frustrations with her. Their relationship is obviously founded on a deep trust and respect for each other. Furthermore, their bond is clearly the result of a long-standing association which had been established when both were much younger. The two appear to be of the same or a similar age and would thus understand each other better, easing their conversation since they would essentially be speaking 'the same language.'
Portia comfortably expresses her displeasure with the current line of suitors to Nerissa, who encourages her mistress to speak openly and freely. Nerissa obviously realises that Portia needs an outlet to vent her frustrations and she becomes her 'shoulder to cry on.' Added to this is also Portia's vexation about her father's will which she deems 'the will of a living daughter curbed by the will of a dead father.' As a caring and understanding friend, Nerissa avoids being too critical and advises Portia to rather see the good in her father's determination.
The depth of the two girls' relationship is further illustrated by the fact that Portia asks Nerissa to accompany her to Venice, in disguise, to act in Antony's defence against Shylock's malicious demand. This is further emphasised by the fact that Nerissa acts as her assistant during the trial and both girls encounter a similar complication with regard to the rings they had each given their husbands, with similar outcomes.
We learn of Bassanio's interest in Portia when he asks Antonio for financial assistance to woo the wealthy heiress. Bassanio wishes to stand an equal chance to win her hand but needs money to do so. He tells Antonio:
In Belmont is a lady richly left;
And she is fair, and, fairer than that word,
Of wondrous virtues: sometimes from her eyes
I did receive fair speechless messages:
Her name is Portia, nothing undervalued
To Cato's daughter, Brutus' Portia:
Nor is the wide world ignorant of her worth, ...
He is clearly infatuated with this lady 'of wondrous virtues.' He also makes it clear that Portia has shown interest in him since she had given him 'speechless messages' with her eyes. Portia must have given him loving looks which have convinced him that he stands a good chance of winning her hand.
When Bassanio arrives at Belmont, he is encouraged by Portia to choose the right casket. She makes it pertinently clear that she desires Bassanio and would stay his visit if she has to. She also mentions that she would have advised him how to choose if it had not meant the forfeiture of her inheritance. She intentionally wastes time in conversing with him so that she may enjoy his company for longer because he might just choose wrongly.
Bassanio expresses similar sentiments and states that he is tormented by the idea of losing his love if he should make the wrong choice. He expresses his love for Portia but wants to get done with the task of choosing a casket so that his torment may cease. he, fortunately, chooses the right chest and is soon married to his beloved. The wedding is a rushed ceremony because he has just been informed about the trouble Antonio is in.
Further evidence of Portia's love for him is found in the fact that not only does she offer to settle Antonio's debt many times over, so that he may be at peace, but also that she ventures to Venice, in disguise, to defend Antonio. In the end, the two are united back at Belmont to enjoy their nuptial pleasures and live a life of love and prosperity.