What does Antonio say about his sadness?

The first words of Antonio, and the play, indicate that he does not really know why he is so sad,

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.

Salarnio speculates that Antonio's sadness stems from his loss of ships or his love life.