Scene 1 Act 1

We can only speculate on exactly why Antonio is sad. He says he has lost his mirth and is depressed.

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.

Some speculate Antonio is lonely. Antonio has a connection with Bassanio and he feels he is going to lose him to Portia. Is Antonio in love with Bassanio? This depends on how you interpret the play.