Antonio is a generous, willing, loyal friend and an understanding devoted friend. He is willing to come to Bassanio's aid for what some might call a flimsy reason. As a businessman, he has all his money tied up in one venture, which shows questionable business wisdom. His generosity and another view of his questionable business wisdom is revealed when he agrees to act as co-signer to "secure" a loan with Bassanio that Bassanio might find in the open marketplace.
As it happens, Bassanio secures a loan with Shylock, a Jewish money lender who charges interest on loans, which is called usury, a practice with which our credit cards make us very familiar. Antonio and Shylock are on extremely unfriendly terms with each other because Antonio has publicly denounced Shylock for his money lending practices. Antonio is the opposite of Shylock because, along with his merchant enterprises, Antonio also lends money but does it without charging a fee for the loan. Shylock of course resents Antonio's treatment and has deep seated ill-will toward him.
Bassanio's loan with Shylock points out that Antoinio is unrealistic and has no foresight. (1) He publicly makes an enemy of Shylock who is not acting illegally in lending money and whose services aid even his best friend Bassanio. (2) He presumes on good fortune and good timing and therefore agrees to impossible terms on the loan specifying a "pound of flesh" for the forfeit of the loan if not paid on time. In addition to Antonio's unrealistic attitude and lack of foresight, he also has a huge ego in that he won't humble himself to Shylock and get rational terms for the secural of Bassanio's loan.
Antonio admits to a deep sadness but offers no explanation for it. Critics suggest that it relates to his imprudent financial decision to loan money at no charge, which means that he has little capital for operating his merchandising business. This lack of capital (because it is all lent out) requires him to put all his eggs in one basket, so to speak, by having his prosperity all tied up in one ship. We know that this is so because the inciting incident is that he has no money to loan Bassanio.
His composure when facing Shylock's terms of the pound of flesh to be delivered up for the failure to make the repayment may represent Christian equanimity, as critics suggst, or it could represent the continuance of the despondency of sadness that has turned to despair at the loss of his ship.