According to Salarino, what is the reason behind Antonio's sadness?

It's the opening scene of The Merchant of Venice. Antonio enters with his two friends, Solanio and Salarino. Antonio is the first to speak:

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. (Act I Scene I).

Antonio is feeling down, but doesn't have the faintest idea why. So Salarino suggests possible reasons to account for Antonio's sadness:


Your mind is tossing on the ocean,
There, where your argosies with portly sail



Not unreasonably, Salarino thinks that Antonio might be worried over his merchant ships. After all, it's a highly risky business, with much valuable cargo being lost at sea. Solanio concurs, but Antonio doesn't think that that is what's really bugging him. He doesn't have all his eggs in one basket, so to speak, hence there's no real danger of his being wiped out financially.
 
If Antonio isn't depressed over his financial situation, suggests Salario, then it must be because he is in love. Antonio quickly rejects his friend's suggestion.