The two men share a very deep bond and seem to have a deep affection for one another. It has been suggested by many commentators that the melancholy that Antonio experiences at the beginning of the play is due to the fact that he realizes that he might lose his companion to another, for Bassanio wishes to travel to Belmont and try his luck in the lottery to win Portia's hand in marriage. Others are of the opinion that the two men share more than just a brotherly affection for each other and are romantically involved.
Whatever the case may be, it is obvious that they are very close. The depth of their relationship is mirrored in the closeness shared between Portia and her hand-maiden, Nerissa. They are not only just friends, but are confidantes as well.
It is clear that there is a relationship of trust between the two men because Bassanio approaches Antonio to ask him for a huge loan so that he may visit Belmont and compete against Portia's many other suitors. Having money will not only enable him to travel there but also make him look the part, since all Portia's suitors are gentleman with titles and status.
It is also obvious that Bassanio, who comes across as a wastrel, has grown dependent on Antonio's generosity, as he himself declares in Act 1, scene 1:
'Tis not unknown to you, Antonio,
How much I have disabled mine estate,
By something showing a more swelling port
Than my faint means would grant continuance:...
...To you, Antonio,
I owe the most, in money and in love,
Antonio has no qualms in helping his friend. At this juncture, though, he is unable to help his desperate companion since he has no ready cash available. All his merchant ships are at sea. He, however, does urge him to seek a loan from a moneylender in Venice, a task he will also undertake. He will then stand surety for the debt.
Antonio's allegiance to Bassanio is more than evident when he enters into a risky agreement with the Jewish moneylender, Shylock. He is more than willing to sign a bond with the Jew so that Bassanio may realize his dream. The bond conditions are harsh and require that Antonio should sacrifice a pound of his flesh to Shylock if he should forfeit. The agreement is for three thousand ducats to be paid in three months. Antonio signs the bond against Bassanio's advice.
Antonio's remarkably philanthropic gesture clearly denotes the love he has for the reckless Bassanio. The loan is granted and Bassanio soon sets off for Belmont. Once there, he is fortunate enough to choose the right casket and wins the beautiful Portia's hand in marriage. Just before their marriage, however, he receives the devastating news that Shylock has had Antonio arrested for forfeiting on the bond. The Christian merchant had lost his ships at sea and was penniless. As he reads the letter concerning this issue, Portia sees him grow deathly pale and expresses her concern in Act 3, scene 2:
There are some shrewd contents in yon same paper,
That steals the colour from Bassanio's cheek:
Some dear friend dead; else nothing in the world
Could turn so much the constitution
Of any constant man.
On her inquiry, Bassanio tells her about Antonio's troubles and confirms the intensity of their friendship. The fact that Bassanio is so distressed about Antonio's misfortune is pertinent evidence that he cares much for his older friend. He calls Antonio his 'dearest friend' and immediately decides, with Portia's permission, to go to Venice where he can assist him.
Further proof of the strong bond between the two men is found in Bassanio's desperate attempts to have Shylock withdraw his claim for a pound of Antonio's flesh. He tries to negotiate with him, but the recalcitrant moneylender is not moved, even when he is offered twice the amount of the original loan.
In the end, though, Antonio is released because of Portia's intelligent intervention, and Shylock is severely sanctioned. On their return to Belmont, Antonio once again displays loyalty to his friend when he offers up his soul to vouch for Bassanio's loyalty to Portia. He says in Act 5, scene 1:
I once did lend my body for his wealth;
Which, but for him that had your husband's ring,
Had quite miscarried: I dare be bound again,
My soul upon the forfeit, that your lord
Will never more break faith advisedly.
To copy Shakespeare's style in Romeo and Juliet, one can only say:
Never was there a greater tale to make all know
about the depth of friendship between Antonio and his Bassanio.