In Shakespeare's Merchant of Venice, why does Antonio say that the reasons Salarino and Salanio give for his sadness are not the real reasons?

Antonio never actually reveals the reason for his sadness. The audience must infer it from his behavior. He seems to believe his melancholy is innate and mysterious. It frustrates both Antonio and his friends Salarino and Salanio, who suggest he is worrying about the safety of his goods: “Your mind is tossing on the ocean; / There, where your argosies with portly sail, / … Do overpeer the petty traffickers.” They both agree that this would cause them lots of anxiety and distress. Antonio says that his “ventures are not in one bottom trusted,” they are spread out, so he does not concern himself about their loss.

Salarino, perhaps jokingly, says that Antonio is in love. Antonio dismisses this with, “Fie, fie!” They conclude that it is simply in Antonio’s nature to be sad: “Nature hath framed strange fellows in her time.” Perhaps Antonio’s rebuttal of their suggestions means that he enjoys being sad. It could be that he suffers from what would now be diagnosed as clinical depression. Alternately, he is embarrassed to reveal the true reason for his heartache. His relationship with Bassanio gives some clues. Antonio showers Bassanio with money and gifts and even risks his life for Bassanio’s financial well-being, without regret. When Bassanio leaves to woo Portia for marriage, Salarino describes the emotion with which Antonio bids Bassanio farewell. Salanio replies, “I think he only loves the world for him.” Antonio may be in love with Bassanio, consciously or unconsciously, but Bassanio plans to marry the wealthy and beautiful Portia.