It is patently clear from the text that Antonio and Bassanio share a deep friendship. Bassanio trusts that Antonio will assist him in any predicament for he has obviously done so in the past. Their friendship is obvious as one can gauge from their initial conversation:
...To you, Antonio,
I owe the most, in money and in love,
And from your love I have a warranty
To unburden all my plots and purposes
How to get clear of all the debts I owe.
I pray you, good Bassanio, let me know it;
And if it stand, as you yourself still do,
Within the eye of honour, be assured,
My purse, my person, my extremest means,
Lie all unlock'd to your occasions.
The generous Antonio is prepared to help his friend in whatever situation he may find himself. Bassanio now faces another predicament for he wishes to woo the wealthy heiress, Portia, but he lacks the means of a fair chance against his other competitors since they are well off and men of some stature or title. He has too many unpaid debts and has now approached his friend for assistance.
Furthermore, it becomes evident that Bassanio had relied on his friend in the past, without always making good on his promises, but Antonio is more than willing to help him once more.
...do but say to me what I should do
That in your knowledge may by me be done,
And I am prest unto it: therefore, speak.
When Bassanio tells him of his current dilemma, Antonio informs him that he has invested his money in merchandise which he is transporting by sea and that he does not have any ready cash available. He does, however, ask him to use his good name to obtain credit and he would do the same.
Go, presently inquire, and so will I,
Where money is, and I no question make
To have it of my trust or for my sake.
Bassanio is given the freedom to obtain a loan using Antonio as security, something with which he will have no issue. Such magnanimity is indeed rare and reflects the depth of their relationship.
Bassanio eventually obtains a loan from the moneylender, Shylock, and Antonio signs a bond as security. He is prepared to take the risk, even if the terms state that Shylock may cut out a pound of his flesh if he should forfeit. Antonio signs the deed against Bassanio's advice.
We later learn that Antonio's ships have been wrecked and that he is bankrupt and, therefore, unable to repay the loans. Shylock demands restitution and asks the duke for judgement. When Bassanio, who is at Belmont after he had successfully won Portia's hand, hears about his friend's dilemma, he is utterly distraught, so much so, that he turns pale. His shocked response to the news moves Portia to enquire about his reaction.
With leave, Bassanio: I am half yourself,
And I must freely have the half of anything
That this same paper brings you.
Bassanio then informs her about Antonio's desperate situation and she vows to help. Bassanio's trauma in this instance further illustrates how much he cared for his friend. He was quite desperate to help him overcome his precarious circumstance. It is to this end that he decides to rush to Venice after a quick wedding ceremony.
On his arrival in Venice, Bassanio attempts to sway Shylock from his malicious purpose by offering him six thousand ducats, twice the original loan amount, but Shylock remains steadfast. He wants to exact his revenge on Antonio and nothing will move him. Bassanio tries to bring Antonio some comfort and says:
Good cheer, Antonio! What, man, courage yet!
The Jew shall have my flesh, blood, bones and all,
Ere thou shalt lose for me one drop of blood.
His statement once again provides evidence of the depth of his commitment to his friend. He is prepared to give his own life before he would allow Antonio to be harmed. He later also declares, not knowing that he is in Portia's presence since she is disguised as a lawyer:
Antonio, I am married to a wife
Which is as dear to me as life itself;
But life itself, my wife, and all the world,
Are not with me esteem'd above thy life:
I would lose all, ay, sacrifice them all
Here to this devil, to deliver you.
Portia's clever intervention forces Shylock to withdraw his demand and he is prepared to accept thrice the amount of the loan which Bassanio immediately offers. His wife, however, has another ace up her sleeve and uses Venetian law to damn Shylock. Since he purposely wanted to harm a Venetian citizen, the court finds against him.
Later, when they have all returned to Belmont, Portia plays on Bassanio's emotions with regard to her ring that he gave away, thinking that he had given it as a gift to the eminent lawyer who had saved his friend's life. Antonio, who feels responsible for the altercation between the two, intervenes and says:
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.
In this instance, Antonio once more displays his love for his friend, offering to bond himself and forfeit his soul to guarantee that Bassanio would never again break an oath. The matter is resolved, though, when Portia declares the truth - that she and Nerissa had, in fact, been the lawyer and his clerk to whom the rings had been given.
The two men truly exemplify the expression: "A friend in need is a friend indeed."