Why is Antonio sad in "The Merchant of Venice"?He says it's nothing to do with money, or love.

Shakespeare never directly tells us the answer! As you suggest, his friends suggest that he's worried about his ships - his merchandise. But Antonio responds

My ventures are not in one bottom trusted,
Nor to one place; nor is my whole estate
Upon the fortune of this present year
Therefore my merchandise makes me not sad.

So it's not to do with business. Salanio has another idea:

Why, then you are in love.

ANTONIO
Fie, fie!

How do you paraphrase "Fie, fie!"? Our paraphrase here has "nonsense, nonsense". "Yeah, yeah" might be a better option. It implies a negative response without actually stating one.

Then Bassanio enters and everyone exits, leaving him with "better company". And Shakespeare is keen to tell us that Antonio already knows what Bassanio is coming to talk about:

Well; tell me now, what lady is the same
To whom you swore a secret pilgrimage
That you today promis'd to tell me of?

Might Antonio be said because Bassanio is thinking of marrying? Let me state again, it's not absolutely in the text, but I think it's implied. And look what Antonio says to Bassanio in the trial:

Commend me to your honourable wife...
Say, how I lov'd you, speak me fair in death;
And, when the tale is told, bid her be judge
Whether Bassanio had not once a love.

Perhaps it is love after all.