Antonio's melancholy is the topic of conversation at the opening of the play, Act I, scene i. He begins with these words: "In sooth, I know not why I am so sad," and though his comrades try to attribute his emotional state to the fortunes of his ships full of merchandise on an uncertain sea or love, Antonio repudiates both, remaining puzzled by his own emotions:
It wearies me. You say it wearies you.
But how I caught it, found it, or came by it,
What stuff 'tis made of...
I am to learn.
And such a want-wit sadness makes of me,
That I have much ado to know myself.
Antonio confesses that he really doesn't have any idea why he's sad, and it seems that he's been in this funk for awhile, since he mentions that both he and his comrades are "weary" of it.
Finally, one of his comrades concludes that he is "sad/Because [he is] not merry." This sadness without cause is often referred to as melancholy, and Antonio accepts his role in the world:
I hold the world but as the world, Gratiano,
A stage where every man must play a part,
And mine a sad one.
melancholy man Q(9/5): one answer to the "why" part of the question is Hamlet's speech "the purpose of playing.........is, to hold , as 't were, the mirror up to nature." Given the contrasts and conflicts in the play one might quote MND: "The best in this kind are but shadows; and the worst are no worse, if imagination amend them." We then have other comments such as the argument that Antonio's particular sadness is due to his offensive behavior towards Shylock or perhaps he has reached age four or five in the melancholy Jaques' list(As You Like It2.7).