What conclusion does Salarino come to when Antonio remarks that he is not sad because of love in The Merchant of Venice?

Salarino, as the other answer states, assumes that Antonio's melancholy arises from his worries about his merchandise at sea. A good deal of money and risk is riding on Anotnio's investment. Antonio, however, insists he is not worried about that. He says that, because he has several ships out collecting merchandise, he feels sure that at least one of them will return safely, which is all he needs to have happen. As we would say today, he has diversified his risk. Salarino suggests that love is causing Antonio's sadness. This Antonio also denies.
However, Antonio's eagerness to lend Bassanio money to woo Portia supports the idea that he is in love with this younger man. It would also make sense that Antonio would feel depressed at the idea of losing this close friend to marriage. Antonio might not realize why he is sad, but we, as an audience, are led to perceive what he does not.
The scene skillfully imparts a good deal of information through a casual dialogue, giving the viewer or reader important background for understanding the story that unfolds.

In the opening scene of the play, Antonio comments that he is depressed but does not know the reason why he is in such a despondent, melancholy mood. Salarino believes that Antonio is worried about his merchant ships, which are at sea. Salarino thinks Antonio is preoccupied with the well-being of his ships on the treacherous seas and fears that he will lose his investments and capital in a rough storm. Salarino also mentions that if he were in Antonio's position, he would think about his ships every time he blew on his soup, looked at an hourglass full of sand, or saw any stones that would remind him of the jagged rocks on the shorelines. Salarino concludes by saying,

I know Antonio is sad to think upon his merchandise (Shakespeare, 1.1.40).

When Antonio insists that he is not worried about his merchant ships, Solanio suggests that he is depressed because of unrequited love. Antonio once again denies love as being the cause of his melancholy, and Salarino does not make any additional remarks regarding Antonio's comment before he exits the scene with Solanio.