Antonio and Bassanio appear to have a family connection, since Salanio refers to Bassanio as Antonio's "most noble kinsman." This, however, is secondary to their deep friendship, to which Antonio immediately alludes as soon as he realizes that Bassanio is going to ask him for money:

My purse, my person, my extremest means,
Lie all unlock'd to your occasions.

This is something more than ordinary friendship and is closer to a marriage vow, in which the couple declare that all their possessions are held in common. It is clear that Bassanio has borrowed money from Antonio before without paying it back, but Antonio remains eager to lend or, indeed, to give his friend whatever he needs. His only reproach is that Bassanio is not more direct and confident in asking for his help, knowing the love Antonio has for him.

You know me well, and herein spend but time
To wind about my love with circumstance;
And out of doubt you do me now more wrong
In making question of my uttermost
Than if you had made waste of all I have.

Again, this is more than ordinary friendship, and Antonio's language sometimes leads audiences to question the nature of the friendship between the two men. As in ancient Greece, intense romantic relationships between men were common in Elizabethan England, meaning that Shakespeare would not have thought of Antonio's love for Bassanio as "homosexual" (a word which did not exist at the time). There is no indication of a physical relationship between the two, and Bassanio is actually asking for his friend's help in pursuing a woman.

It also seems that Antonio's feelings are deeper and stronger than Bassanio's. The relationship might, therefore, be characterized as a deep and passionate friendship in which Antonio is the more emotionally invested. Bassanio, while he loves his friend, is less emotionally attached but is, to some extent, materially dependent on Antonio.

Antonio and Bassanio are the closest of friends, and it is their relationship in William Shakespeare’s The Merchant of Venice that provides the foundation of the play’s two-pronged plot. Antonio is the titular merchant whose business arrangements weigh heavily on his demeanor.  While a risky business strategy occupies his time and thoughts, however, it is his friend’s seeming predicament regarding the beautiful, wealthy Portia that most troubles him. An early indication of the closeness of these two characters is provided early in the play when Antonio speaks with two of his friends, Salanio and Salarino. Salanio and Salarino are questioning Antonio regarding the latter’s sadness when Salanio observes the approach of Bassanio, prompting his comment, “Here comes Bassanio, your most noble kinsman.” Following their departure, as well as that of Lorenzo and Gratiano, Antonio and Bassanio engage in the conversation that leads Antonio into the fateful arrangement with Shylock, the Jewish moneylender:

BASSANIO:

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.

ANTONIO

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 strength of this relationship, the depth of the love these two men hold for one another, is such that Antonio will agree to pay Shylock a pound of his flesh should he fail to repay the moneylender within the agreed upon time. The purpose of the loan, of course, is to facilitate Bassanio’s pursuit of Portia.

Antonio is the merchant who is very generous, particularly with his good friends. Bassanio needs money in order to woo Portia. Even though all of Antonio's money is tied up in products yet to be sold, he agrees to help Bassanio because they are such close friends. Antonio is such a loyal friend to Bassanio that he agrees to borrow money from Shylock with the stipulation that Shylock can exact a pound of flesh from Antonio if he does not repay the loan within the agreed-upon time frame. 

Bassanio is more impulsive and irresponsible (especially with money). And although he is loyal to Antonio, he does take advantage of his generosity; the attempt to woo Portia (and thereby inherit the vast amount of money of her estate by marrying her) is a gamble that he is willing to take with Antonio's money and the pound of flesh. However, Bassanio does reveal that he is more than a self-indulgent gambler who takes advantage of his friend. He is a very loyal friend just as Antonio is to him. He offers Shylock twice the amount he'd loaned to Antonio. Shylock refuses and then Bassanio offers to give his own flesh instead of Antonio's: 

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. (IV.i.113-15) 

In the end, neither Antonio nor Bassanio are cut because a disguised Portia discovers a technicality in the agreement that no blood can be shed. In the earlier case, Bassanio needs money and Antonio helps him. In the later case, Antonio is facing a potentially fatal injury and Bassanio offers to take his place.