In Shakespeare's The Merchant of Venice, how does Antonio describe his sad mood in Act 1, Scene 1?

The Merchant of Venice opens by describing Antonio's sadness, and indeed, his melancholic state becomes a recurrence throughout the rest of the play. Antonio describes his sadness as a mystery - he claims not to know why he is sad - but close examination of the play's language and Antonio's motivations throughout the play may reveal the source of his melancholy.

To illustrate, consider the opening lines of the play, in which Antonio states,

"In sooth I know not why I am so sad.It wearies me, you say it wearies you;
But how I caught it, found it, or came by it,
What stuff 'tis made of, whereof it is born,
I am to learn..." (1.1.1-5)

In these first lines of the play, Antonio states that he is sad and does not know why. It is important that Shakespeare inserts these lines as the first in Act 1, Scene 1, as they establish that Antonio is a melancholy character and that his sadness will come to define much of his character as the play continues. Furthermore, although Antonio does not know why he is sad, his friends think they have an answer: Solanio thinks his "affections" are with his "hopes abroad," (1.1.16-17) or in other words, that Antonio is worried about his mercantile ventures. As Antonio is the play's titular "Merchant of Venice," it might make sense that he is worried about his monetary ventures, and indeed, his friend Salerio agrees that "Antonio / Is sad to think upon his merchandise" (1.1.39-40).

However, Antonio denies that he is worried about his money, as his "ventures are not in one bottom trusted" (1.1.42); in other words, he has many ships abroad and does not need to worry about the fate of any one particular ship. To this rebuttal, Solanio responds that the only other explanation is that Antonio is in love, to which Antonio replies "Fie, fie!" (an expression of disgust or outrage, here meant to imply denial) (1.1.47).

Of course, as with many of the characters' statements in the play, we must take Antonio's statements with a grain of salt. His denial of being in love is immediately contrasted with his avowals of love for his friend Bassanio later in the scene, when Bassanio seeks a loan from Antonio in order to court a woman, Portia. To Bassanio's request, he says, "My purse, my person, my extremest means / Lie unlocked to your occasions" (1.1.138-139). In other words, Antonio "loves" Bassanio, and is willing to do anything for him; and in this case, Antonio is willing to help Bassanio acquire a large loan to court Portia, even if it means losing his best friend to marriage. Indeed, many critics have identified this loss of Bassanio's love as the source of Antonio's sadness.

So in summary, Antonio opens the play by saying he is sad, but does not know why. This fact is important for Antonio's characterization, as he remains a melancholic character in the play. Furthermore, Solanio and Solerio suggest Antonio is sad about his ships, or that he is in love, but Antonio denies both accusations. However, it is likely that the true source of his sadness is that Bassanio is seeking a wife, and Bassanio's eventual marriage will erode the close friendship between the two men.