In The Merchant of Venice, it is quite clear from the outset that Antonio and Bassanio are dear friends. Modern audiences do not have the same acceptance or appreciation for their friendship and wonder why Antonio allows himself to be exposed by someone claiming to be his friend. However, Bassanio confides in Antonio who is waiting to hear the news of the lady and the "secret pilgrimage" he refers to in Act I, scene i (120) and Bassanio expresses his sincere regret at having "disabled mine estate" (123). He has promised to always tell Antonio the truth and so he admits that he has lost everything, including monies that Antonio has previously given him. It does seem that he is taking advantage of Antonio's apparent patience for his unacceptable behavior but the audience will see how he changes his mind about borrowing the money from Shylock when he hears what the forfeit is and that Antonio is expected to offer "an equal pound of your fair flesh" (I.iii.144). Antonio convinces Bassanio that it will never come to this so he is still prepared to sign the bond. Bassanio indicates that "I like not fair terms and a villain's mind"(174) and distrusts Shylock. He is persuaded by Antonio's confidence that his ships will arrive a month before the due date.
When matters take a turn for the worse for Antonio, Bassanio has been able to secure his marriage to Portia. However, when he hears of Antonio's plight, he is distraught even though Antonio's status no longer affects his chances and he could quite easily have not concerned himself with Antonio's present circumstances. So devastated is he that even Portia decides to risk everything for her new husband by first offering him everything that is hers and ultimately by posing as a man and setting up Shylock to ensure that the debt against Antonio is reversed. For Portia to go to such lengths when she sees her husband's distress must mean that Bassanio really is genuine.
One would think that Bassanio would have a strained relationship with Antonio. However, Antonio (in Act 4) does not seem to blame Bassanio for his life being in danger even though the dramatic court scene is partially a result of Bassanio's poor spending habits. In fact,while Antonio is frightened by Shylock's insistence on cutting out a pound of Antonio's flesh, he doesn't seem to be upset with Bassanio at all.
In reference to Bassanio's motivation for rushing to Antonio's defense and using Portia's money to try to satisfy Shylock's bond, I think that Shakespeare intends for the relationship between Bassanio and Antonio to be sincere. While modern audiences are suspicious of Bassanio's motives, by the time the action of Act 4 occurs, Bassanio does not need to borrow more money from Antonio because he has just married the extremely wealthy Portia who bestowed upon Bassanio all her possessions, status, and wealth. Most likely, Bassanio helps Antonio not just because he cares for him as a friend but also because he feels guilty for asking Antonio to lend him money in the first place because that leads to the bond with Shylock.
It is true that Bassanio is taking money from Antonio but he loves Antonio from heart. Not only because he is lending him money but also because they are very good friends. He don't want that Antonio should sign that unfair bond because he don't want to risk his friends life . But when Antonio says that he is confident that his ships will return from the sea before the decided time Antonio signed the bond. After taking money from shylock bassanio went to Belmont to marry portia. There bassanio got the message about his friend that Antonio is now in custody because of not paying his debts. Here we see that bassanio is so kind to his friend that he left his new wife there and came to Antonio to rescue him from the trap of shylock. Even bassanio's wife is so kind that she also risk all her wealth only for his husband's friend. She also appeared as a lawyer to rescue Antonio in venice.
Bassanio made so many offers to shylock to leave his friend's life but shylock did not accept his single offer and only demands Antonio's pound of flesh and that is also from nearest to his heart.
When shylock was not accepting any offer Portia turned the case towards shylock that he can take the pound of flesh but without dropping any drop of blood because taking blood is not mentioned in the bond . Therefore with this shylock left his bond and Antonio's life was saved.