The opening lines of this play quickly establish the character of Antonio, the Merchant of the title, by explaining a sadness that he feels in himself but does not understand:
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;
And such a want-wit sadness makes of me,
That I have much ado to know myself.
Critics have hotly debated the reasons for this melancholy. Antonio denies the reasons suggested to him by his friends, Solanio and Salerio, namely that he is worried about his latest shipping venture, and secondly that he is in love, but given our knowledge about the play we can perhaps doubt Antonio's protestations that these are not the reasons for his melancholy. Certainly in the opening scene we are introduced to the world of upper class Venetian merchants, whose wealth was made or broken on the success or otherwise of various highly risky shipping ventures, as is made clear later in the play.
Secondly, it is strongly suggested in the play that Antonio is deeply in love with Bassanio (what other motive is their for his willingness to pledge his "pound of flesh" to Shylock?). Given his love, perhaps he was aware of Bassanio's plan to court and marry Portia, and by so doing, end their intimacy.
Lastly, perhaps we can explain his melancholy by a sense of foreboding he has of the imminent disaster that is about to fall upon him during the action of the play. Whatever the reason, these lines contrast Antonio's character from the light-hearted levity of his fellow merchants, Solerio and Solanio.