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It maybe because his ships are out in the sea and he is afraid whether it will wreck.... But another main reason for Antonio's sadness would be that his beat frnd bassanio is going to Belmont to win portia's hand in marriage. Antonio is sad that he would miss his frnd bassanio's company
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At the beginning of the play, we find that Antonio is in a melancholy mood. He doesn't know the reason for his sadness. His friends, Salerio and Solanio state some reasons which Antonio denies. But probable reasons must be that Antonio was very anxious to know what Bassanio wanted to say. He would also miss his company if he wins Portia by lottery and resides in Belmont.